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<h1 class="title">Data stored in TREXIO</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#orgab1a474">1. Metadata (metadata group)</a></li>
<li><a href="#orgf9513f8">2. System</a>
<ul>
<li><a href="#org4261204">2.1. Nucleus (nucleus group)</a></li>
<li><a href="#org9e86efb">2.2. Cell (cell group)</a></li>
<li><a href="#org6bbf2f9">2.3. Periodic boundary calculations (pbc group)</a></li>
<li><a href="#org6a05989">2.4. Electron (electron group)</a></li>
<li><a href="#org5ebc7bb">2.5. Ground or excited states (state group)</a></li>
</ul>
</li>
<li><a href="#orgf127689">3. Basis functions</a>
<ul>
<li><a href="#orgef5a2b0">3.1. Basis set (basis group)</a>
<ul>
<li><a href="#org13514b7">3.1.1. Gaussian and Slater-type orbitals</a></li>
<li><a href="#orgafb8b49">3.1.2. Numerical orbitals</a></li>
<li><a href="#orgf68af51">3.1.3. Plane waves</a></li>
<li><a href="#org4982f58">3.1.4. Oscillating orbitals</a></li>
<li><a href="#org407980d">3.1.5. Data definitions</a></li>
<li><a href="#org4c0b901">3.1.6. Example</a></li>
</ul>
</li>
<li><a href="#org8357b3c">3.2. Effective core potentials (ecp group)</a>
<ul>
<li><a href="#org0b9bcae">3.2.1. Example</a></li>
</ul>
</li>
<li><a href="#org94128ac">3.3. Numerical integration grid (grid group)</a></li>
</ul>
</li>
<li><a href="#orga5647fc">4. Orbitals</a>
<ul>
<li><a href="#org03841e3">4.1. Atomic orbitals (ao group)</a>
<ul>
<li><a href="#ao_one_e">4.1.1. One-electron integrals (<code>ao_1e_int</code> group)</a></li>
<li><a href="#ao_two_e">4.1.2. Two-electron integrals (<code>ao_2e_int</code> group)</a></li>
</ul>
</li>
<li><a href="#org5120a0d">4.2. Molecular orbitals (mo group)</a>
<ul>
<li><a href="#org09f5486">4.2.1. One-electron integrals (<code>mo_1e_int</code> group)</a></li>
<li><a href="#orgd6cf9e7">4.2.2. Two-electron integrals (<code>mo_2e_int</code> group)</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#org63bf171">5. Multi-determinant information</a>
<ul>
<li><a href="#orgfff9e85">5.1. Slater determinants (determinant group)</a></li>
<li><a href="#org40ac910">5.2. Configuration state functions (csf group)</a></li>
<li><a href="#org6b24ce0">5.3. Amplitudes (amplitude group)</a></li>
<li><a href="#org91a7262">5.4. Reduced density matrices (rdm group)</a></li>
</ul>
</li>
<li><a href="#org05dc8e5">6. Correlation factors</a>
<ul>
<li><a href="#orgaa6089b">6.1. Jastrow factor (jastrow group)</a>
<ul>
<li><a href="#org4d858c9">6.1.1. CHAMP</a></li>
<li><a href="#org1e66881">6.1.2. Mu</a></li>
<li><a href="#org0605eff">6.1.3. Table of values</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#org4bba6a8">7. Quantum Monte Carlo data (qmc group)</a></li>
</ul>
</div>
</div>
<p>
For simplicity, the singular form is always used for the names of
groups and attributes, and all data are stored in atomic units.
The dimensions of the arrays in the tables below are given in
column-major order (as in Fortran), and the ordering of the dimensions
is reversed in the produced <code>trex.json</code> configuration file as the
library is written in C.
</p>
<p>
Dimensions are given both in row-major <code>[]</code> and column-major <code>()</code>
formats. Pick the one adapted to the programming language in which
you use TREXIO (Numpy is by default row-major, and Fortran is column-major).
In the column-major representation, <code>A(i,j)</code> and <code>A(i+1,j)</code> are
contiguous in memory. In the row-major representation, <code>A[i,j]</code> and
<code>A[i,j+1]</code> are contiguous.
</p>
<div id="outline-container-orgab1a474" class="outline-2">
<h2 id="orgab1a474"><span class="section-number-2">1</span> Metadata (metadata group)</h2>
<div class="outline-text-2" id="text-1">
<p>
As we expect TREXIO files to be archived in open-data repositories,
we give the possibility to the users to store some metadata inside
the files. We propose to store the list of names of the codes which
have participated to the creation of the file, a list of authors of
the file, and a textual description.
</p>
<table id="orga3cb84c" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>code_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of codes used to produce the file</td>
</tr>
<tr>
<td class="org-left"><code>code</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left"><code>[metadata.code_num]</code></td>
<td class="org-left"><code>(metadata.code_num)</code></td>
<td class="org-left">Names of the codes used</td>
</tr>
<tr>
<td class="org-left"><code>author_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of authors of the file</td>
</tr>
<tr>
<td class="org-left"><code>author</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left"><code>[metadata.author_num]</code></td>
<td class="org-left"><code>(metadata.author_num)</code></td>
<td class="org-left">Names of the authors of the file</td>
</tr>
<tr>
<td class="org-left"><code>package_version</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">TREXIO version used to produce the file</td>
</tr>
<tr>
<td class="org-left"><code>description</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Text describing the content of file</td>
</tr>
<tr>
<td class="org-left"><code>unsafe</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left"><code>1</code>: true, <code>0</code>: false</td>
</tr>
</tbody>
</table>
<p>
<b><b>Note:</b></b> The <code>unsafe</code> attribute of the <code>metadata</code> group indicates
whether the file has been previously opened with <code>'u'</code> mode. It is
automatically written in the file upon the first unsafe opening. If
the user has checked that the TREXIO file is valid (e.g. using
<code>trexio-tools</code>) after unsafe operations, then the <code>unsafe</code> attribute
value can be manually overwritten (in unsafe mode) from <code>1</code> to <code>0</code>.
</p>
</div>
</div>
<div id="outline-container-orgf9513f8" class="outline-2">
<h2 id="orgf9513f8"><span class="section-number-2">2</span> System</h2>
<div class="outline-text-2" id="text-2">
</div>
<div id="outline-container-org4261204" class="outline-3">
<h3 id="org4261204"><span class="section-number-3">2.1</span> Nucleus (nucleus group)</h3>
<div class="outline-text-3" id="text-2-1">
<p>
The nuclei are considered as fixed point charges. Coordinates are
given in Cartesian \((x,y,z)\) format.
</p>
<table id="orgb5949da" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
<th scope="col" class="org-left">&#xa0;</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of nuclei</td>
<td class="org-left">&#xa0;</td>
</tr>
<tr>
<td class="org-left"><code>charge</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[nucleus.num]</code></td>
<td class="org-left"><code>(nucleus.num)</code></td>
<td class="org-left">Charges of the nuclei</td>
<td class="org-left">&#xa0;</td>
</tr>
<tr>
<td class="org-left"><code>coord</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[nucleus.num, 3]</code></td>
<td class="org-left"><code>(3, nucleus.num)</code></td>
<td class="org-left">Coordinates of the atoms</td>
<td class="org-left">&#xa0;</td>
</tr>
<tr>
<td class="org-left"><code>label</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left"><code>[nucleus.num]</code></td>
<td class="org-left"><code>(nucleus.num)</code></td>
<td class="org-left">Atom labels</td>
<td class="org-left">&#xa0;</td>
</tr>
<tr>
<td class="org-left"><code>point_group</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Symmetry point group</td>
<td class="org-left">&#xa0;</td>
</tr>
<tr>
<td class="org-left"><code>repulsion</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Nuclear repulsion energy</td>
<td class="org-left">&#xa0;</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org9e86efb" class="outline-3">
<h3 id="org9e86efb"><span class="section-number-3">2.2</span> Cell (cell group)</h3>
<div class="outline-text-3" id="text-2-2">
<p>
3 Lattice vectors to define a box containing the system, for example
used in periodic calculations.
</p>
<table id="org8ccc696" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>a</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[3]</code></td>
<td class="org-left"><code>(3)</code></td>
<td class="org-left">First real space lattice vector</td>
</tr>
<tr>
<td class="org-left"><code>b</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[3]</code></td>
<td class="org-left"><code>(3)</code></td>
<td class="org-left">Second real space lattice vector</td>
</tr>
<tr>
<td class="org-left"><code>c</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[3]</code></td>
<td class="org-left"><code>(3)</code></td>
<td class="org-left">Third real space lattice vector</td>
</tr>
<tr>
<td class="org-left"><code>g_a</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[3]</code></td>
<td class="org-left"><code>(3)</code></td>
<td class="org-left">First reciprocal space lattice vector</td>
</tr>
<tr>
<td class="org-left"><code>g_b</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[3]</code></td>
<td class="org-left"><code>(3)</code></td>
<td class="org-left">Second reciprocal space lattice vector</td>
</tr>
<tr>
<td class="org-left"><code>g_c</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[3]</code></td>
<td class="org-left"><code>(3)</code></td>
<td class="org-left">Third reciprocal space lattice vector</td>
</tr>
<tr>
<td class="org-left"><code>two_pi</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left"><code>0</code> or <code>1</code>. If <code>two_pi=1</code>, \(2\pi\) is included in the reciprocal vectors.</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org6bbf2f9" class="outline-3">
<h3 id="org6bbf2f9"><span class="section-number-3">2.3</span> Periodic boundary calculations (pbc group)</h3>
<div class="outline-text-3" id="text-2-3">
<table id="org69f21fd" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>periodic</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left"><code>1</code>: true or <code>0</code>: false</td>
</tr>
<tr>
<td class="org-left"><code>k_point_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of $k$-points</td>
</tr>
<tr>
<td class="org-left"><code>k_point</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[3]</code></td>
<td class="org-left"><code>(3)</code></td>
<td class="org-left">$k$-point sampling</td>
</tr>
<tr>
<td class="org-left"><code>k_point_weight</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[pbc.k_point_num]</code></td>
<td class="org-left"><code>(pbc.k_point_num)</code></td>
<td class="org-left">$k$-point weight</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org6a05989" class="outline-3">
<h3 id="org6a05989"><span class="section-number-3">2.4</span> Electron (electron group)</h3>
<div class="outline-text-3" id="text-2-4">
<p>
The chemical system consists of nuclei and electrons, where the
nuclei are considered as fixed point charges with Cartesian
coordinates. The wave function is stored in the spin-free
formalism, and therefore, it is necessary for the user to
explicitly store the number of electrons with spin up
(\(N_\uparrow\)) and spin down (\(N_\downarrow\)). These numbers
correspond to the normalization of the spin-up and spin-down
single-particle reduced density matrices.
</p>
<p>
We consider wave functions expressed in the spin-free formalism, where
the number of &uarr; and &darr; electrons is fixed.
</p>
<table id="org867ddba" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of electrons</td>
</tr>
<tr>
<td class="org-left"><code>up_num</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of &uarr;-spin electrons</td>
</tr>
<tr>
<td class="org-left"><code>dn_num</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of &darr;-spin electrons</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org5ebc7bb" class="outline-3">
<h3 id="org5ebc7bb"><span class="section-number-3">2.5</span> Ground or excited states (state group)</h3>
<div class="outline-text-3" id="text-2-5">
<p>
This group contains information about excited states. Since only a
single state can be stored in a TREXIO file, it is possible to store
in the main TREXIO file the names of auxiliary files containing the
information of the other states.
</p>
<p>
The <code>file_name</code> and <code>label</code> arrays have to be written only for the
main file, e.g. the one containing the ground state wave function
together with the basis set parameters, molecular orbitals,
integrals, etc.
The <code>id</code> and <code>current_label</code> attributes need to be specified for each file.
</p>
<table id="org09fbbdf" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of states (including the ground state)</td>
</tr>
<tr>
<td class="org-left"><code>id</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Index of the current state (0 is ground state)</td>
</tr>
<tr>
<td class="org-left"><code>energy</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Energy of the current state</td>
</tr>
<tr>
<td class="org-left"><code>current_label</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Label of the current state</td>
</tr>
<tr>
<td class="org-left"><code>label</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left"><code>[state.num]</code></td>
<td class="org-left"><code>(state.num)</code></td>
<td class="org-left">Labels of all states</td>
</tr>
<tr>
<td class="org-left"><code>file_name</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left"><code>[state.num]</code></td>
<td class="org-left"><code>(state.num)</code></td>
<td class="org-left">Names of the TREXIO files linked to the current one (i.e. containing data for other states)</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div id="outline-container-orgf127689" class="outline-2">
<h2 id="orgf127689"><span class="section-number-2">3</span> Basis functions</h2>
<div class="outline-text-2" id="text-3">
</div>
<div id="outline-container-orgef5a2b0" class="outline-3">
<h3 id="orgef5a2b0"><span class="section-number-3">3.1</span> Basis set (basis group)</h3>
<div class="outline-text-3" id="text-3-1">
</div>
<div id="outline-container-org13514b7" class="outline-4">
<h4 id="org13514b7"><span class="section-number-4">3.1.1</span> Gaussian and Slater-type orbitals</h4>
<div class="outline-text-4" id="text-3-1-1">
<p>
We consider here basis functions centered on nuclei. Hence, it is
possibile to define <i>dummy atoms</i> to place basis functions in
arbitrary positions.
</p>
<p>
The atomic basis set is defined as a list of shells. Each shell \(s\) is
centered on a center \(A\), possesses a given angular momentum \(l\) and a
radial function \(R_s\). The radial function is a linear combination of
\(N_{\text{prim}}\) <i>primitive</i> functions that can be of type
Slater (\(p=1\)) or Gaussian (\(p=2\)),
parameterized by exponents \(\gamma_{ks}\) and coefficients \(a_{ks}\):
\[
R_s(\mathbf{r}) = \mathcal{N}_s \vert\mathbf{r}-\mathbf{R}_A\vert^{n_s}
\sum_{k=1}^{N_{\text{prim}}} a_{ks}\, f_{ks}(\gamma_{ks},p)\,
\exp \left( - \gamma_{ks}
\vert \mathbf{r}-\mathbf{R}_A \vert ^p \right).
\]
</p>
<p>
In the case of Gaussian functions, \(n_s\) is always zero.
</p>
<p>
Different codes normalize functions at different levels. Computing
normalization factors requires the ability to compute overlap
integrals, so the normalization factors should be written in the
file to ensure that the file is self-contained and does not need the
client program to have the ability to compute such integrals.
</p>
<p>
Some codes assume that the contraction coefficients are for a linear
combination of <i>normalized</i> primitives. This implies that a normalization
constant for the primitive \(ks\) needs to be computed and stored. If
this normalization factor is not required, \(f_{ks}=1\).
</p>
<p>
Some codes assume that the basis function are normalized. This
implies the computation of an extra normalization factor, \(\mathcal{N}_s\).
If the the basis function is not considered normalized, \(\mathcal{N}_s=1\).
</p>
<p>
All the basis set parameters are stored in one-dimensional arrays.
</p>
</div>
</div>
<div id="outline-container-orgafb8b49" class="outline-4">
<h4 id="orgafb8b49"><span class="section-number-4">3.1.2</span> Numerical orbitals</h4>
<div class="outline-text-4" id="text-3-1-2">
<p>
Trexio supports numerical atom centered orbitals. The implementation is
based on the approach of FHI-aims [Blum, V. et al; Ab initio molecular
simulations with numeric atom-centered orbitals; Computer Physics
Communications 2009]. These orbitals are
defined by the atom they are centered on, their angular momentum and a
radial function \(R_s\), which is of the form
\[
R_s(\mathbf{r}) = \mathcal{N}_s \frac{u_i(\mathbf{r})}{r^{-n_s}}.
\]
Where \(u_i(\mathbf{r})\) is numerically tabulated on a dense logarithmic
grid. It is constructed to vanish for any \(\mathbf{r}\)
outside of the grid. The reference points are stored in <code>nao_grid_r</code>
and <code>nao_grid_phi</code>. Additionaly, a separate spline for the first and second
derivative of \(u(\mathbf{r})\) can be stored in <code>nao_grid_grad</code> and <code>nao_grid_lap</code>.
Storing them in this form allows to calculate the actual first and
second derivatives easily as follows:
</p>
<p>
\[
\frac{\partial \phi}{\partial x} = \frac{x}{r^2}\left( u^\prime\left(r\right) - \frac{u\left(r\right)}{r}\right)
\]
\[
\frac{\partial^2 \phi}{\partial x^2} = \frac{1}{r^3}\left(x^2 u^{\prime\prime}(r) + \left( 3x^2-r^2\right) \left( \frac{u(r)}{r^2} - \frac{u'(r)}{r}\right) \right)
\]
</p>
<p>
The index of the first data point for each shell is stored in
<code>nao_grid_start</code>, the number of data points per spline is stored
in <code>nao_grid_size</code> for convenience.
</p>
<p>
What kind of spline is used can be provided in the <code>interpolator_kind</code> field.
For example, FHI-aims uses a cubic spline, so the <code>interpolator_kind</code> is
\"Polynomial\" and the <code>interp_coeff_cnt</code> is \(4\). In this case, the first
interpolation coefficient per data point is the absolute term, the second is for
the linear term etc.
The interpolation coefficients for the wave function are given in the
<code>interpolator_phi</code> array. The <code>interpolator_grad</code> and <code>interpolator_lap</code>
arrays provide a spline for the gradient and Laplacian, respectively.
The argument passed to the interpolants is on the logarithmic scale of
the reference points: If the argument is an integer \(i\), the interpolant
will return the value of \(u(\mathbf{r})\) at the $i$th reference point.
A radius is converted to this scale by (note the zero-indexing)
\[
i_{\log} = \frac{1}{c} \cdot \log \left( \frac{r}{r_0} \right)
\]
where
\[
c = \log\left(\frac{r_1}{r_0}\right)
\]
For convenience, this conversion and functions to evaluate the splines
are provided with trexio. Since these implementations are not adapted to
a specific software architecture, a programm using these orbitals should
reimplement them with consideration for its specific needs.
</p>
</div>
</div>
<div id="outline-container-orgf68af51" class="outline-4">
<h4 id="orgf68af51"><span class="section-number-4">3.1.3</span> Plane waves</h4>
<div class="outline-text-4" id="text-3-1-3">
<p>
A plane wave is defined as
</p>
<p>
\[
\chi_j(\mathbf{r}) = \exp \left( -i \mathbf{G}_j \cdot \mathbf{r} \right)
\]
</p>
<p>
The basis set is defined as the array of $k$-points in the
reciprocal space \(\mathbf{G}_j\), defined in the <code>pbc</code> group. The
kinetic energy cutoff <code>e_cut</code> is the only input data relevant to
plane waves.
</p>
</div>
</div>
<div id="outline-container-org4982f58" class="outline-4">
<h4 id="org4982f58"><span class="section-number-4">3.1.4</span> Oscillating orbitals</h4>
<div class="outline-text-4" id="text-3-1-4">
<p>
Basis functions can be made oscillating as
\[
R_s(\mathbf{r}) = \mathcal{N}_s \vert\mathbf{r}-\mathbf{R}_A\vert^{n_s}
\sum_{k=1}^{N_{\text{prim}}} a_{ks}\, f_{ks}(\gamma_{ks},p)\,
\exp \left( - \gamma_{ks}
\vert \mathbf{r}-\mathbf{R}_A \vert ^p \right)\, \cos \left( \beta_{ks} \vert \mathbf{r}-\mathbf{R}_A \vert ^q \right)
\]
</p>
<p>
Oscillation kind can be:
</p>
<ul class="org-ul">
<li><code>Cos1</code> for \(q=1\)</li>
<li><code>Cos2</code> for \(q=2\)</li>
</ul>
</div>
</div>
<div id="outline-container-org407980d" class="outline-4">
<h4 id="org407980d"><span class="section-number-4">3.1.5</span> Data definitions</h4>
<div class="outline-text-4" id="text-3-1-5">
<table id="org0672623" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>type</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Type of basis set: "Gaussian", "Slater", "Numerical" or "PW" for plane waves</td>
</tr>
<tr>
<td class="org-left"><code>prim_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Total number of primitives</td>
</tr>
<tr>
<td class="org-left"><code>shell_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Total number of shells</td>
</tr>
<tr>
<td class="org-left"><code>nao_grid_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Total number of grid points for numerical orbitals</td>
</tr>
<tr>
<td class="org-left"><code>interp_coeff_cnt</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of coefficients for the numerical orbital interpolator</td>
</tr>
<tr>
<td class="org-left"><code>nucleus_index</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[basis.shell_num]</code></td>
<td class="org-left"><code>(basis.shell_num)</code></td>
<td class="org-left">One-to-one correspondence between shells and atomic indices</td>
</tr>
<tr>
<td class="org-left"><code>shell_ang_mom</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left"><code>[basis.shell_num]</code></td>
<td class="org-left"><code>(basis.shell_num)</code></td>
<td class="org-left">One-to-one correspondence between shells and angular momenta</td>
</tr>
<tr>
<td class="org-left"><code>shell_factor</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.shell_num]</code></td>
<td class="org-left"><code>(basis.shell_num)</code></td>
<td class="org-left">Normalization factor of each shell (\(\mathcal{N}_s\))</td>
</tr>
<tr>
<td class="org-left"><code>r_power</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left"><code>[basis.shell_num]</code></td>
<td class="org-left"><code>(basis.shell_num)</code></td>
<td class="org-left">Power to which \(r\) is raised (\(n_s\))</td>
</tr>
<tr>
<td class="org-left"><code>nao_grid_start</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[basis.shell_num]</code></td>
<td class="org-left"><code>(basis.shell_num)</code></td>
<td class="org-left">Index of the first data point for a given numerical orbital</td>
</tr>
<tr>
<td class="org-left"><code>nao_grid_size</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left"><code>[basis.shell_num]</code></td>
<td class="org-left"><code>(basis.shell_num)</code></td>
<td class="org-left">Number of data points per numerical orbital</td>
</tr>
<tr>
<td class="org-left"><code>shell_index</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[basis.prim_num]</code></td>
<td class="org-left"><code>(basis.prim_num)</code></td>
<td class="org-left">One-to-one correspondence between primitives and shell index</td>
</tr>
<tr>
<td class="org-left"><code>exponent</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.prim_num]</code></td>
<td class="org-left"><code>(basis.prim_num)</code></td>
<td class="org-left">Exponents of the primitives (\(\gamma_{ks}\))</td>
</tr>
<tr>
<td class="org-left"><code>exponent_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.prim_num]</code></td>
<td class="org-left"><code>(basis.prim_num)</code></td>
<td class="org-left">Imaginary part of the exponents of the primitives (\(\gamma_{ks}\))</td>
</tr>
<tr>
<td class="org-left"><code>coefficient</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.prim_num]</code></td>
<td class="org-left"><code>(basis.prim_num)</code></td>
<td class="org-left">Coefficients of the primitives (\(a_{ks}\))</td>
</tr>
<tr>
<td class="org-left"><code>coefficient_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.prim_num]</code></td>
<td class="org-left"><code>(basis.prim_num)</code></td>
<td class="org-left">Imaginary part of the coefficients of the primitives (\(a_{ks}\))</td>
</tr>
<tr>
<td class="org-left"><code>oscillation_arg</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.prim_num]</code></td>
<td class="org-left"><code>(basis.prim_num)</code></td>
<td class="org-left">Additional argument to have oscillating orbitals (\(\beta_{ks}\))</td>
</tr>
<tr>
<td class="org-left"><code>oscillation_kind</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Kind of Oscillating function:"Cos1" or "Cos2"</td>
</tr>
<tr>
<td class="org-left"><code>prim_factor</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.prim_num]</code></td>
<td class="org-left"><code>(basis.prim_num)</code></td>
<td class="org-left">Normalization coefficients for the primitives (\(f_{ks}\))</td>
</tr>
<tr>
<td class="org-left"><code>e_cut</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Energy cut-off for plane-wave calculations</td>
</tr>
<tr>
<td class="org-left"><code>nao_grid_radius</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.nao_grid_num]</code></td>
<td class="org-left"><code>(basis.nao_grid_num)</code></td>
<td class="org-left">Radii of grid points for numerical orbitals</td>
</tr>
<tr>
<td class="org-left"><code>nao_grid_phi</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.nao_grid_num]</code></td>
<td class="org-left"><code>(basis.nao_grid_num)</code></td>
<td class="org-left">Wave function values for numerical orbitals</td>
</tr>
<tr>
<td class="org-left"><code>nao_grid_grad</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.nao_grid_num]</code></td>
<td class="org-left"><code>(basis.nao_grid_num)</code></td>
<td class="org-left">Radial gradient of numerical orbitals</td>
</tr>
<tr>
<td class="org-left"><code>nao_grid_lap</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.nao_grid_num]</code></td>
<td class="org-left"><code>(basis.nao_grid_num)</code></td>
<td class="org-left">Laplacian of numerical orbitals</td>
</tr>
<tr>
<td class="org-left"><code>interpolator_kind</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Kind of spline, e.g. "Polynomial"</td>
</tr>
<tr>
<td class="org-left"><code>interpolator_phi</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.nao_grid_num, basis.interp_coeff_cnt]</code></td>
<td class="org-left"><code>(basis.interp_coeff_cnt, basis.nao_grid_num)</code></td>
<td class="org-left">Coefficients for numerical orbital interpolation function</td>
</tr>
<tr>
<td class="org-left"><code>interpolator_grad</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.nao_grid_num, basis.interp_coeff_cnt]</code></td>
<td class="org-left"><code>(basis.interp_coeff_cnt, basis.nao_grid_num)</code></td>
<td class="org-left">Coefficients for numerical orbital gradient interpolation function</td>
</tr>
<tr>
<td class="org-left"><code>interpolator_lap</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[basis.nao_grid_num, basis.interp_coeff_cnt]</code></td>
<td class="org-left"><code>(basis.interp_coeff_cnt, basis.nao_grid_num)</code></td>
<td class="org-left">Coefficients for numerical orbital laplacian interpolation function</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org4c0b901" class="outline-4">
<h4 id="org4c0b901"><span class="section-number-4">3.1.6</span> Example</h4>
<div class="outline-text-4" id="text-3-1-6">
<p>
For example, consider H<sub>2</sub> with the following basis set (in GAMESS
format), where both the AOs and primitives are considered normalized:
</p>
<pre class="example">
HYDROGEN
S 5
1 3.387000E+01 6.068000E-03
2 5.095000E+00 4.530800E-02
3 1.159000E+00 2.028220E-01
4 3.258000E-01 5.039030E-01
5 1.027000E-01 3.834210E-01
S 1
1 3.258000E-01 1.000000E+00
S 1
1 1.027000E-01 1.000000E+00
P 1
1 1.407000E+00 1.000000E+00
P 1
1 3.880000E-01 1.000000E+00
D 1
1 1.057000E+00 1.000000E+00
</pre>
<p>
In TREXIO representaion we have:
</p>
<pre class="example">
type = "Gaussian"
prim_num = 20
shell_num = 12
# 6 shells per H atom
nucleus_index =
[ 0, 0, 0, 0, 0, 0,
1, 1, 1, 1, 1, 1 ]
# 3 shells in S (l=0), 2 in P (l=1), 1 in D (l=2)
shell_ang_mom =
[ 0, 0, 0, 1, 1, 2,
0, 0, 0, 1, 1, 2 ]
# no need to renormalize shells
shell_factor =
[ 1., 1., 1., 1., 1., 1.,
1., 1., 1., 1., 1., 1. ]
# 5 primitives for the first S shell and then 1 primitive per remaining shells in each H atom
shell_index =
[ 0, 0, 0, 0, 0, 1, 2, 3, 4, 5,
6, 6, 6, 6, 6, 7, 8, 9, 10, 11 ]
# parameters of the primitives (10 per H atom)
exponent =
[ 33.87, 5.095, 1.159, 0.3258, 0.1027, 0.3258, 0.1027, 1.407, 0.388, 1.057,
33.87, 5.095, 1.159, 0.3258, 0.1027, 0.3258, 0.1027, 1.407, 0.388, 1.057 ]
coefficient =
[ 0.006068, 0.045308, 0.202822, 0.503903, 0.383421, 1.0, 1.0, 1.0, 1.0, 1.0,
0.006068, 0.045308, 0.202822, 0.503903, 0.383421, 1.0, 1.0, 1.0, 1.0, 1.0 ]
prim_factor =
[ 1.0006253235944540e+01, 2.4169531573445120e+00, 7.9610924849766440e-01
3.0734305383061117e-01, 1.2929684417481876e-01, 3.0734305383061117e-01,
1.2929684417481876e-01, 2.1842769845268308e+00, 4.3649547399719840e-01,
1.8135965626177861e+00, 1.0006253235944540e+01, 2.4169531573445120e+00,
7.9610924849766440e-01, 3.0734305383061117e-01, 1.2929684417481876e-01,
3.0734305383061117e-01, 1.2929684417481876e-01, 2.1842769845268308e+00,
4.3649547399719840e-01, 1.8135965626177861e+00 ]
</pre>
</div>
</div>
</div>
<div id="outline-container-org8357b3c" class="outline-3">
<h3 id="org8357b3c"><span class="section-number-3">3.2</span> Effective core potentials (ecp group)</h3>
<div class="outline-text-3" id="text-3-2">
<p>
An effective core potential (ECP) \(V_A^{\text{ECP}}\) replacing the
core electrons of atom \(A\) can be expressed as
\[
V_A^{\text{ECP}} =
V_{A \ell_{\max}+1} +
\sum_{\ell=0}^{\ell_{\max}}
\delta V_{A \ell}\sum_{m=-\ell}^{\ell} | Y_{\ell m} \rangle \langle Y_{\ell m} |
\]
</p>
<p>
The first term in this equation is attributed to the local channel, while
the remaining terms correspond to non-local channel projections. \(\ell_{\max}\)
refers to the maximum angular momentum in the non-local component of the ECP.
The functions \(\delta V_{A \ell}\) and \(V_{A \ell_{\max}+1}\) are parameterized as:
</p>
\begin{eqnarray}
\delta V_{A \ell}(\mathbf{r}) &=&
\sum_{q=1}^{N_{q \ell}}
\beta_{A q \ell}\, |\mathbf{r}-\mathbf{R}_{A}|^{n_{A q \ell}}\,
e^{-\alpha_{A q \ell} |\mathbf{r}-\mathbf{R}_{A}|^2 } \nonumber\\
V_{A \ell_{\max}+1}(\mathbf{r}) &=& -\frac{Z_\text{eff}}{|\mathbf{r}-\mathbf{R}_{A}|}+\delta V_{A \ell_{\max}+1}(\mathbf{r})
\end{eqnarray}
<p>
where \(Z_\text{eff}\) is the effective nuclear charge of the center.
</p>
<p>
See <a href="http://dx.doi.org/10.1063/1.4984046">http://dx.doi.org/10.1063/1.4984046</a> or <a href="https://doi.org/10.1063/1.5121006">https://doi.org/10.1063/1.5121006</a> for more info.
</p>
<table id="orge821c10" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>max_ang_mom_plus_1</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left"><code>[nucleus.num]</code></td>
<td class="org-left"><code>(nucleus.num)</code></td>
<td class="org-left">\(\ell_{\max}+1\), one higher than the max angular momentum in the removed core orbitals</td>
</tr>
<tr>
<td class="org-left"><code>z_core</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left"><code>[nucleus.num]</code></td>
<td class="org-left"><code>(nucleus.num)</code></td>
<td class="org-left">Number of core electrons to remove per atom</td>
</tr>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Total number of ECP functions for all atoms and all values of \(\ell\)</td>
</tr>
<tr>
<td class="org-left"><code>ang_mom</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left"><code>[ecp.num]</code></td>
<td class="org-left"><code>(ecp.num)</code></td>
<td class="org-left">One-to-one correspondence between ECP items and the angular momentum \(\ell\)</td>
</tr>
<tr>
<td class="org-left"><code>nucleus_index</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[ecp.num]</code></td>
<td class="org-left"><code>(ecp.num)</code></td>
<td class="org-left">One-to-one correspondence between ECP items and the atom index</td>
</tr>
<tr>
<td class="org-left"><code>exponent</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ecp.num]</code></td>
<td class="org-left"><code>(ecp.num)</code></td>
<td class="org-left">\(\alpha_{A q \ell}\) all ECP exponents</td>
</tr>
<tr>
<td class="org-left"><code>coefficient</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ecp.num]</code></td>
<td class="org-left"><code>(ecp.num)</code></td>
<td class="org-left">\(\beta_{A q \ell}\) all ECP coefficients</td>
</tr>
<tr>
<td class="org-left"><code>power</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left"><code>[ecp.num]</code></td>
<td class="org-left"><code>(ecp.num)</code></td>
<td class="org-left">\(n_{A q \ell}\) all ECP powers</td>
</tr>
</tbody>
</table>
<p>
There might be some confusion in the meaning of the \(\ell_{\max}\).
It can be attributed to the maximum angular momentum occupied in
the core orbitals, which are removed by the ECP. On the other
hand, it can be attributed to the maximum angular momentum of the
ECP that replaces the core electrons.
<b>Note</b>, that the latter \(\ell_{\max}\) is always higher by 1 than the former.
</p>
<p>
<b>Note for developers</b>: avoid having variables with similar prefix
in their name. The HDF5 back end might cause issues due to the way
<code>find_dataset</code> function works. For example, in the ECP group we
use <code>max_ang_mom</code> and not <code>ang_mom_max</code>. The latter causes issues
when written before the <code>ang_mom</code> array in the TREXIO file.
<b>Update</b>: in fact, the aforementioned issue has only been observed
when using HDF5 version 1.10.4 installed via <code>apt-get</code>. Installing
the same version from the <code>conda-forge</code> channel and running it in
an isolated <code>conda</code> environment works just fine. Thus, it seems to
be a bug in the <code>apt</code>-provided package.
If you encounter the aforementioned issue, please report it to our
<a href="https://github.com/TREX-CoE/trexio/issues">issue tracker on GitHub</a>.
</p>
</div>
<div id="outline-container-org0b9bcae" class="outline-4">
<h4 id="org0b9bcae"><span class="section-number-4">3.2.1</span> Example</h4>
<div class="outline-text-4" id="text-3-2-1">
<p>
For example, consider H<sub>2</sub> molecule with the following
<a href="https://pseudopotentiallibrary.org/recipes/H/ccECP/H.ccECP.gamess">effective core potential</a>
(in GAMESS input format for the H atom):
</p>
<pre class="example">
H-ccECP GEN 0 1
3
1.00000000000000 1 21.24359508259891
21.24359508259891 3 21.24359508259891
-10.85192405303825 2 21.77696655044365
1
0.00000000000000 2 1.000000000000000
</pre>
<p>
In TREXIO representation this would be:
</p>
<pre class="example">
num = 8
# lmax+1 per atom
max_ang_mom_plus_1 = [ 1, 1 ]
# number of core electrons to remove per atom
zcore = [ 0, 0 ]
# first 4 ECP elements correspond to the first H atom ; the remaining 4 elements are for the second H atom
nucleus_index = [
0, 0, 0, 0,
1, 1, 1, 1
]
# 3 first ECP elements correspond to potential of the P orbital (l=1), then 1 element for the S orbital (l=0) ; similar for the second H atom
ang_mom = [
1, 1, 1, 0,
1, 1, 1, 0
]
# ECP quantities that can be attributed to atoms and/or angular momenta based on the aforementioned ecp_nucleus and ecp_ang_mom arrays
coefficient = [
1.00000000000000, 21.24359508259891, -10.85192405303825, 0.00000000000000,
1.00000000000000, 21.24359508259891, -10.85192405303825, 0.00000000000000
]
exponent = [
21.24359508259891, 21.24359508259891, 21.77696655044365, 1.000000000000000,
21.24359508259891, 21.24359508259891, 21.77696655044365, 1.000000000000000
]
power = [
-1, 1, 0, 0,
-1, 1, 0, 0
]
</pre>
</div>
</div>
</div>
<div id="outline-container-org94128ac" class="outline-3">
<h3 id="org94128ac"><span class="section-number-3">3.3</span> Numerical integration grid (grid group)</h3>
<div class="outline-text-3" id="text-3-3">
<p>
In some applications, such as DFT calculations, integrals have to
be computed numerically on a grid. A common choice for the angular
grid is the one proposed by Lebedev and Laikov
[Russian Academy of Sciences Doklady Mathematics, Volume 59, Number 3, 1999, pages 477-481].
For the radial grids, many approaches have been developed over the years.
</p>
<p>
The structure of this group is adapted for the <a href="https://github.com/dftlibs/numgrid">numgrid</a> library.
Feel free to submit a PR if you find missing options/functionalities.
</p>
<table id="orgc9e2624" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>description</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Details about the used quadratures can go here</td>
</tr>
<tr>
<td class="org-left"><code>rad_precision</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Radial precision parameter (not used in some schemes like Krack-Köster)</td>
</tr>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of grid points</td>
</tr>
<tr>
<td class="org-left"><code>max_ang_num</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Maximum number of angular grid points (for pruning)</td>
</tr>
<tr>
<td class="org-left"><code>min_ang_num</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Minimum number of angular grid points (for pruning)</td>
</tr>
<tr>
<td class="org-left"><code>coord</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[grid.num]</code></td>
<td class="org-left"><code>(grid.num)</code></td>
<td class="org-left">Discretized coordinate space</td>
</tr>
<tr>
<td class="org-left"><code>weight</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[grid.num]</code></td>
<td class="org-left"><code>(grid.num)</code></td>
<td class="org-left">Grid weights according to a given partitioning (e.g. Becke)</td>
</tr>
<tr>
<td class="org-left"><code>ang_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of angular integration points (if used)</td>
</tr>
<tr>
<td class="org-left"><code>ang_coord</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[grid.ang_num]</code></td>
<td class="org-left"><code>(grid.ang_num)</code></td>
<td class="org-left">Discretized angular space (if used)</td>
</tr>
<tr>
<td class="org-left"><code>ang_weight</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[grid.ang_num]</code></td>
<td class="org-left"><code>(grid.ang_num)</code></td>
<td class="org-left">Angular grid weights (if used)</td>
</tr>
<tr>
<td class="org-left"><code>rad_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of radial integration points (if used)</td>
</tr>
<tr>
<td class="org-left"><code>rad_coord</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[grid.rad_num]</code></td>
<td class="org-left"><code>(grid.rad_num)</code></td>
<td class="org-left">Discretized radial space (if used)</td>
</tr>
<tr>
<td class="org-left"><code>rad_weight</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[grid.rad_num]</code></td>
<td class="org-left"><code>(grid.rad_num)</code></td>
<td class="org-left">Radial grid weights (if used)</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div id="outline-container-orga5647fc" class="outline-2">
<h2 id="orga5647fc"><span class="section-number-2">4</span> Orbitals</h2>
<div class="outline-text-2" id="text-4">
</div>
<div id="outline-container-org03841e3" class="outline-3">
<h3 id="org03841e3"><span class="section-number-3">4.1</span> Atomic orbitals (ao group)</h3>
<div class="outline-text-3" id="text-4-1">
<p>
AOs are defined as
</p>
<p>
\[
\chi_i (\mathbf{r}) = \mathcal{N}_i'\, P_{\eta(i)}(\mathbf{r})\, R_{s(i)} (\mathbf{r})
\]
</p>
<p>
where \(i\) is the atomic orbital index, \(P\) refers to either
polynomials or spherical harmonics, and \(s(i)\) specifies the shell
on which the AO is expanded.
</p>
<p>
\(\eta(i)\) denotes the chosen angular function. The AOs can be
expressed using real spherical harmonics or polynomials in Cartesian
coordinates. In the case of real spherical harmonics, the AOs are
ordered as \(0, +1, -1, +2, -2, \dots, + m, -m\) (see <a href="https://en.wikipedia.org/wiki/Table_of_spherical_harmonics#Real_spherical_harmonics">Wikipedia</a>). In
the case of polynomials, the canonical (or alphabetical) ordering is
used,
</p>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
</colgroup>
<tbody>
<tr>
<td class="org-left">\(p\)</td>
<td class="org-left">\(p_x, p_y, p_z\)</td>
</tr>
<tr>
<td class="org-left">\(d\)</td>
<td class="org-left">\(d_{xx}, d_{xy}, d_{xz}, d_{yy}, d_{yz}, d_{zz}\)</td>
</tr>
<tr>
<td class="org-left">\(f\)</td>
<td class="org-left">\(f_{xxx}, f_{xxy}, f_{xxz}, f_{xyy}, f_{xyz}\),</td>
</tr>
<tr>
<td class="org-left">&#xa0;</td>
<td class="org-left">\(f_{xzz}, f_{yyy}, f_{yyz}, f_{yzz}, f_{zzz}\)</td>
</tr>
<tr>
<td class="org-left">\(\vdots\)</td>
<td class="org-left">&#xa0;</td>
</tr>
</tbody>
</table>
<p>
Note that for \(p\) orbitals in spherical coordinates, the ordering
is \(0,+1,-1\) which corresponds to \(p_z, p_x, p_y\).
</p>
<p>
\(\mathcal{N}_i'\) is a normalization factor that allows for different
normalization coefficients within a single shell, as in the GAMESS
convention where each individual function is unit-normalized.
Using GAMESS convention, the normalization factor of the shell
\(\mathcal{N}_d\) in the <code>basis</code> group is appropriate for instance
for the \(d_z^2\) function (i.e.
\(\mathcal{N}_{d}\equiv\mathcal{N}_{z^2}\)) but not for the \(d_{xy}\)
AO, so the correction factor \(\mathcal{N}_i'\) for \(d_{xy}\) in the
<code>ao</code> groups is the ratio \(\frac{\mathcal{N}_{xy}}{\mathcal{N}_{z^2}}\).
</p>
<table id="orgaa310d8" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>cartesian</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left"><code>1</code>: true, <code>0</code>: false</td>
</tr>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Total number of atomic orbitals</td>
</tr>
<tr>
<td class="org-left"><code>shell</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[ao.num]</code></td>
<td class="org-left"><code>(ao.num)</code></td>
<td class="org-left">Basis set shell for each AO</td>
</tr>
<tr>
<td class="org-left"><code>normalization</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num]</code></td>
<td class="org-left"><code>(ao.num)</code></td>
<td class="org-left">Normalization factor \(\mathcal{N}_i\)</td>
</tr>
</tbody>
</table>
</div>
<div id="outline-container-org399f1f9" class="outline-4">
<h4 id="ao_one_e"><span class="section-number-4">4.1.1</span> One-electron integrals (<code>ao_1e_int</code> group)</h4>
<div class="outline-text-4" id="text-ao_one_e">
<ul class="org-ul">
<li>\[ \hat{V}_{\text{ne}} = \sum_{A=1}^{N_\text{nucl}}
\sum_{i=1}^{N_\text{elec}} \frac{-Z_A }{\vert \mathbf{R}_A -
\mathbf{r}_i \vert} \] : electron-nucleus attractive potential,</li>
<li>\[ \hat{T}_{\text{e}} =
\sum_{i=1}^{N_\text{elec}} -\frac{1}{2}\hat{\Delta}_i \] : electronic kinetic energy</li>
<li>\(\hat{h} = \hat{T}_\text{e} + \hat{V}_\text{ne} + \hat{V}_\text{ECP}\) : core electronic Hamiltonian</li>
</ul>
<p>
The one-electron integrals for a one-electron operator \(\hat{O}\) are
\[ \langle p \vert \hat{O} \vert q \rangle \], returned as a matrix
over atomic orbitals.
</p>
<table id="orgafc8b44" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>overlap</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert q \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>kinetic</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{T}_e \vert q \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>potential_n_e</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{V}_{\text{ne}} \vert q \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>ecp</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{V}_{\text{ecp}} \vert q \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>core_hamiltonian</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{h} \vert q \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>overlap_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert q \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>kinetic_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{T}_e \vert q \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>potential_n_e_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{V}_{\text{ne}} \vert q \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>ecp_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{V}_{\text{ECP}} \vert q \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>core_hamiltonian_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num)</code></td>
<td class="org-left">\(\langle p \vert \hat{h} \vert q \rangle\) (imaginary part)</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-orgc3c589a" class="outline-4">
<h4 id="ao_two_e"><span class="section-number-4">4.1.2</span> Two-electron integrals (<code>ao_2e_int</code> group)</h4>
<div class="outline-text-4" id="text-ao_two_e">
<p>
The two-electron integrals for a two-electron operator \(\hat{O}\) are
\[ \langle p q \vert \hat{O} \vert r s \rangle \] in physicists
notation, where \(p,q,r,s\) are indices over atomic orbitals.
</p>
<ul class="org-ul">
<li>\[ \hat{W}_{\text{ee}} = \sum_{i=2}^{N_\text{elec}} \sum_{j=1}^{i-1} \frac{1}{\vert \mathbf{r}_i - \mathbf{r}_j \vert} \] : electron-electron repulsive potential operator.</li>
<li>\[ \hat{W}^{lr}_{\text{ee}} = \sum_{i=2}^{N_\text{elec}}
\sum_{j=1}^{i-1} \frac{\text{erf}(\mu\, \vert \mathbf{r}_i -
\mathbf{r}_j \vert)}{\vert \mathbf{r}_i - \mathbf{r}_j \vert} \] : electron-electron long range potential</li>
</ul>
<p>
The Cholesky decomposition of the integrals can also be stored:
</p>
<p>
\[
\langle ij | kl \rangle = \sum_{\alpha} G_{ik\alpha} G_{jl\alpha}
\]
</p>
<table id="org602b38f" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>eri</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[ao.num, ao.num, ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num, ao.num, ao.num)</code></td>
<td class="org-left">Electron repulsion integrals</td>
</tr>
<tr>
<td class="org-left"><code>eri_lr</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[ao.num, ao.num, ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num, ao.num, ao.num)</code></td>
<td class="org-left">Long-range electron repulsion integrals</td>
</tr>
<tr>
<td class="org-left"><code>eri_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors for ERI</td>
</tr>
<tr>
<td class="org-left"><code>eri_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[ao_2e_int.eri_cholesky_num, ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num, ao_2e_int.eri_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the ERI</td>
</tr>
<tr>
<td class="org-left"><code>eri_lr_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors for long range ERI</td>
</tr>
<tr>
<td class="org-left"><code>eri_lr_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[ao_2e_int.eri_lr_cholesky_num, ao.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, ao.num, ao_2e_int.eri_lr_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the long range ERI</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div id="outline-container-org5120a0d" class="outline-3">
<h3 id="org5120a0d"><span class="section-number-3">4.2</span> Molecular orbitals (mo group)</h3>
<div class="outline-text-3" id="text-4-2">
<table id="org29a1290" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>type</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Free text to identify the set of MOs (HF, Natural, Local, CASSCF, <i>etc</i>)</td>
</tr>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of MOs</td>
</tr>
<tr>
<td class="org-left"><code>coefficient</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, mo.num)</code></td>
<td class="org-left">MO coefficients</td>
</tr>
<tr>
<td class="org-left"><code>coefficient_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, ao.num]</code></td>
<td class="org-left"><code>(ao.num, mo.num)</code></td>
<td class="org-left">MO coefficients (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>class</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left"><code>[mo.num]</code></td>
<td class="org-left"><code>(mo.num)</code></td>
<td class="org-left">Choose among: Core, Inactive, Active, Virtual, Deleted</td>
</tr>
<tr>
<td class="org-left"><code>symmetry</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left"><code>[mo.num]</code></td>
<td class="org-left"><code>(mo.num)</code></td>
<td class="org-left">Symmetry in the point group</td>
</tr>
<tr>
<td class="org-left"><code>occupation</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num]</code></td>
<td class="org-left"><code>(mo.num)</code></td>
<td class="org-left">Occupation number</td>
</tr>
<tr>
<td class="org-left"><code>energy</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num]</code></td>
<td class="org-left"><code>(mo.num)</code></td>
<td class="org-left">For canonical MOs, corresponding eigenvalue</td>
</tr>
<tr>
<td class="org-left"><code>spin</code></td>
<td class="org-left"><code>int</code></td>
<td class="org-left"><code>[mo.num]</code></td>
<td class="org-left"><code>(mo.num)</code></td>
<td class="org-left">For UHF wave functions, 0 is \(\alpha\) and 1 is \(\beta\)</td>
</tr>
<tr>
<td class="org-left"><code>k_point</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[mo.num]</code></td>
<td class="org-left"><code>(mo.num)</code></td>
<td class="org-left">For periodic calculations, the \(k\) point to which each MO belongs</td>
</tr>
</tbody>
</table>
<p>
<b><b>Warning</b></b>: <code>mo.num</code> is the total number of MOs stored in the file.
For periodic calculations, this group contains all the MOs
belonging to all \(k\) points so <code>mo.num</code> will be the sum of the
numbers of MOs in each \(k\) point. For UHF wave functions, <code>mo.num</code>
is the number of spin-orbitals, twice the number of spatial orbitals.
</p>
</div>
<div id="outline-container-org09f5486" class="outline-4">
<h4 id="org09f5486"><span class="section-number-4">4.2.1</span> One-electron integrals (<code>mo_1e_int</code> group)</h4>
<div class="outline-text-4" id="text-4-2-1">
<p>
The operators as the same as those defined in the
<a href="#ao_one_e">AO one-electron integrals section</a>. Here, the integrals are given in
the basis of molecular orbitals.
</p>
<table id="org3847da1" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>overlap</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert j \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>kinetic</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{T}_e \vert j \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>potential_n_e</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{V}_{\text{ne}} \vert j \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>ecp</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{V}_{\text{ECP}} \vert j \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>core_hamiltonian</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{h} \vert j \rangle\)</td>
</tr>
<tr>
<td class="org-left"><code>overlap_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert j \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>kinetic_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{T}_e \vert j \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>potential_n_e_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{V}_{\text{ne}} \vert j \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>ecp_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{V}_{\text{ECP}} \vert j \rangle\) (imaginary part)</td>
</tr>
<tr>
<td class="org-left"><code>core_hamiltonian_im</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">\(\langle i \vert \hat{h} \vert j \rangle\) (imaginary part)</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-orgd6cf9e7" class="outline-4">
<h4 id="orgd6cf9e7"><span class="section-number-4">4.2.2</span> Two-electron integrals (<code>mo_2e_int</code> group)</h4>
<div class="outline-text-4" id="text-4-2-2">
<p>
The operators are the same as those defined in the
<a href="#ao_two_e">AO two-electron integrals section</a>. Here, the integrals are given in
the basis of molecular orbitals.
</p>
<table id="org543a25a" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>eri</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num, mo.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo.num, mo.num)</code></td>
<td class="org-left">Electron repulsion integrals</td>
</tr>
<tr>
<td class="org-left"><code>eri_lr</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num, mo.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo.num, mo.num)</code></td>
<td class="org-left">Long-range electron repulsion integrals</td>
</tr>
<tr>
<td class="org-left"><code>eri_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors for ERI</td>
</tr>
<tr>
<td class="org-left"><code>eri_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo_2e_int.eri_cholesky_num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo_2e_int.eri_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the ERI</td>
</tr>
<tr>
<td class="org-left"><code>eri_lr_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors for long range ERI</td>
</tr>
<tr>
<td class="org-left"><code>eri_lr_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo_2e_int.eri_lr_cholesky_num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo_2e_int.eri_lr_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the long range ERI</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
<div id="outline-container-org63bf171" class="outline-2">
<h2 id="org63bf171"><span class="section-number-2">5</span> Multi-determinant information</h2>
<div class="outline-text-2" id="text-5">
</div>
<div id="outline-container-orgfff9e85" class="outline-3">
<h3 id="orgfff9e85"><span class="section-number-3">5.1</span> Slater determinants (determinant group)</h3>
<div class="outline-text-3" id="text-5-1">
<p>
The configuration interaction (CI) wave function \(\Psi\)
can be expanded in the basis of Slater determinants \(D_I\) as follows
</p>
<p>
\[
\Psi = \sum_I C_I D_I
\]
</p>
<p>
For relatively small expansions, a given determinant can be represented as a list of occupied orbitals.
However, this becomes unfeasible for larger expansions and requires more advanced data structures.
The bit field representation is used here, namely a given determinant is represented as \(N_{\text{int}}\)
64-bit integers where j-th bit is set to 1 if there is an electron in the j-th orbital and 0 otherwise.
This gives access to larger determinant expansions by optimising the storage of the determinant lists
in the memory.
</p>
<p>
\[
D_I = \alpha_1 \alpha_2 \ldots \alpha_{n_\uparrow} \beta_1 \beta_2 \ldots \beta_{n_\downarrow}
\]
</p>
<p>
where \(\alpha\) and \(\beta\) denote &uarr;-spin and &darr;-spin electrons, respectively,
\(n_\uparrow\) and \(n_\downarrow\) correspond to <code>electron.up_num</code> and <code>electron.dn_num</code>, respectively.
</p>
<p>
Note: the <code>special</code> attribute is present in the types, meaning that the source node is not
produced by the code generator.
</p>
<p>
An illustration on how to read determinants is presented in the <a href="./examples.html">examples</a>.
</p>
<table id="org5a597f6" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim readonly</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of determinants</td>
</tr>
<tr>
<td class="org-left"><code>list</code></td>
<td class="org-left"><code>int special</code></td>
<td class="org-left"><code>[determinant.num]</code></td>
<td class="org-left"><code>(determinant.num)</code></td>
<td class="org-left">List of determinants as integer bit fields</td>
</tr>
<tr>
<td class="org-left"><code>coefficient</code></td>
<td class="org-left"><code>float buffered</code></td>
<td class="org-left"><code>[determinant.num]</code></td>
<td class="org-left"><code>(determinant.num)</code></td>
<td class="org-left">Coefficients of the determinants from the CI expansion</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org40ac910" class="outline-3">
<h3 id="org40ac910"><span class="section-number-3">5.2</span> Configuration state functions (csf group)</h3>
<div class="outline-text-3" id="text-5-2">
<p>
The configuration interaction (CI) wave function \(\Psi\) can be
expanded in the basis of <a href="https://en.wikipedia.org/wiki/Configuration_state_function">configuration state functions</a> (CSFs)
\(\Psi_I\) as follows
</p>
<p>
\[
\Psi = \sum_I C_I \psi_I.
\]
</p>
<p>
Each CSF \(\psi_I\) is a linear combination of Slater determinants. Slater
determinants are stored in the <code>determinant</code> section. In this group
we store the CI coefficients in the basis of CSFs, and the
matrix \(\langle D_I | \psi_J \rangle\) needed to project the CSFs in
the basis of Slater determinants.
</p>
<table id="org7bfd420" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim readonly</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of CSFs</td>
</tr>
<tr>
<td class="org-left"><code>coefficient</code></td>
<td class="org-left"><code>float buffered</code></td>
<td class="org-left"><code>[csf.num]</code></td>
<td class="org-left"><code>(csf.num)</code></td>
<td class="org-left">Coefficients \(C_I\) of the CSF expansion</td>
</tr>
<tr>
<td class="org-left"><code>det_coefficient</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[csf.num, determinant.num]</code></td>
<td class="org-left"><code>(determinant.num, csf.num)</code></td>
<td class="org-left">Projection on the determinant basis</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org6b24ce0" class="outline-3">
<h3 id="org6b24ce0"><span class="section-number-3">5.3</span> Amplitudes (amplitude group)</h3>
<div class="outline-text-3" id="text-5-3">
<p>
The wave function may be expressed in terms of action of the cluster
operator \(\hat{T}\):
</p>
<p>
\[
\hat{T} = \hat{T}_1 + \hat{T}_2 + \hat{T}_3 + \dots
\]
</p>
<p>
on a reference wave function \(\Psi\), where \(\hat{T}_1\) is the single excitation operator,
</p>
<p>
\[
\hat{T}_1 = \sum_{ia} t_{i}^{a}\, \hat{a}^\dagger_a \hat{a}_i,
\]
</p>
<p>
\(\hat{T}_2\) is the double excitation operator,
</p>
<p>
\[
\hat{T}_2 = \frac{1}{4} \sum_{ijab} t_{ij}^{ab}\, \hat{a}^\dagger_a \hat{a}^\dagger_b \hat{a}_j \hat{a}_i,
\]
</p>
<p>
<i>etc</i>. Indices \(i\), \(j\), \(a\) and \(b\) denote molecular orbital indices.
</p>
<p>
Wave functions obtained with perturbation theory or configuration
interaction are of the form
</p>
<p>
\[ |\Phi\rangle = \hat{T}|\Psi\rangle \]
</p>
<p>
and coupled-cluster wave functions are of the form
</p>
<p>
\[ |\Phi\rangle = e^{\hat{T}}| \Psi \rangle \]
</p>
<p>
The reference wave function is stored using the <code>determinant</code> and/or
<code>csf</code> groups, and the amplitudes are stored using the current group.
The attributes with the <code>exp</code> suffix correspond to exponentialized operators.
</p>
<p>
The order of the indices is chosen such that
</p>
<ul class="org-ul">
<li><code>t(i,a)</code> = \(t_{i}^{a}\).</li>
<li><code>t(i,j,a,b)</code> = \(t_{ij}^{ab}\),</li>
<li><code>t(i,j,k,a,b,c)</code> = \(t_{ijk}^{abc}\),</li>
<li><code>t(i,j,k,l,a,b,c,d)</code> = \(t_{ijkl}^{abcd}\),</li>
<li>\(\dots\)</li>
</ul>
<table id="orgb92d0e5" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>single</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num)</code></td>
<td class="org-left">Single excitation amplitudes</td>
</tr>
<tr>
<td class="org-left"><code>single_exp</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num)</code></td>
<td class="org-left">Exponentialized single excitation amplitudes</td>
</tr>
<tr>
<td class="org-left"><code>double</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num,mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num,mo.num,mo.num)</code></td>
<td class="org-left">Double excitation amplitudes</td>
</tr>
<tr>
<td class="org-left"><code>double_exp</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num,mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num,mo.num,mo.num)</code></td>
<td class="org-left">Exponentialized double excitation amplitudes</td>
</tr>
<tr>
<td class="org-left"><code>triple</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num,mo.num,mo.num,mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num,mo.num,mo.num,mo.num,mo.num)</code></td>
<td class="org-left">Triple excitation amplitudes</td>
</tr>
<tr>
<td class="org-left"><code>triple_exp</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num,mo.num,mo.num,mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num,mo.num,mo.num,mo.num,mo.num)</code></td>
<td class="org-left">Exponentialized triple excitation amplitudes</td>
</tr>
<tr>
<td class="org-left"><code>quadruple</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num)</code></td>
<td class="org-left">Quadruple excitation amplitudes</td>
</tr>
<tr>
<td class="org-left"><code>quadruple_exp</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num]</code></td>
<td class="org-left"><code>(mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num,mo.num)</code></td>
<td class="org-left">Exponentialized quadruple excitation amplitudes</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-org91a7262" class="outline-3">
<h3 id="org91a7262"><span class="section-number-3">5.4</span> Reduced density matrices (rdm group)</h3>
<div class="outline-text-3" id="text-5-4">
<p>
The reduced density matrices are defined in the basis of molecular
orbitals.
</p>
<p>
The &uarr;-spin and &darr;-spin components of the one-body
density matrix are given by
</p>
\begin{eqnarray*}
\gamma_{ij}^{\uparrow} &=& \langle \Psi | \hat{a}^{\dagger}_{j\alpha}\, \hat{a}_{i\alpha} | \Psi \rangle \\
\gamma_{ij}^{\downarrow} &=& \langle \Psi | \hat{a}^{\dagger}_{j\beta} \, \hat{a}_{i\beta} | \Psi \rangle
\end{eqnarray*}
<p>
and the spin-summed one-body density matrix is
\[
\gamma_{ij} = \gamma^{\uparrow}_{ij} + \gamma^{\downarrow}_{ij}
\]
</p>
<p>
The \(\uparrow \uparrow\), \(\downarrow \downarrow\), \(\uparrow \downarrow\) components of the two-body density matrix are given by
</p>
\begin{eqnarray*}
\Gamma_{ijkl}^{\uparrow \uparrow} &=&
\langle \Psi | \hat{a}^{\dagger}_{k\alpha}\, \hat{a}^{\dagger}_{l\alpha} \hat{a}_{j\alpha}\, \hat{a}_{i\alpha} | \Psi \rangle \\
\Gamma_{ijkl}^{\downarrow \downarrow} &=&
\langle \Psi | \hat{a}^{\dagger}_{k\beta}\, \hat{a}^{\dagger}_{l\beta} \hat{a}_{j\beta}\, \hat{a}_{i\beta} | \Psi \rangle \\
\Gamma_{ijkl}^{\uparrow \downarrow} &=&
\langle \Psi | \hat{a}^{\dagger}_{k\alpha}\, \hat{a}^{\dagger}_{l\beta} \hat{a}_{j\beta}\, \hat{a}_{i\alpha} | \Psi \rangle
+ \langle \Psi | \hat{a}^{\dagger}_{l\alpha}\, \hat{a}^{\dagger}_{k\beta} \hat{a}_{i\beta}\, \hat{a}_{j\alpha} | \Psi \rangle \\
\end{eqnarray*}
<p>
and the spin-summed one-body density matrix is
\[
\Gamma_{ijkl} = \Gamma_{ijkl}^{\uparrow \uparrow} +
\Gamma_{ijkl}^{\downarrow \downarrow} + \Gamma_{ijkl}^{\uparrow \downarrow}.
\]
</p>
<p>
The total energy can be computed as:
\[
E = E_{\text{NN}} + \sum_{ij} \gamma_{ij} \langle j|h|i \rangle +
\frac{1}{2} \sum_{ijlk} \Gamma_{ijkl} \langle k l | i j \rangle
\]
</p>
<p>
To compress the storage, the Cholesky decomposition of the RDMs can
be stored:
</p>
<p>
\[
\Gamma_{ijkl} = \sum_{\alpha} G_{ij\alpha} G_{kl\alpha}
\]
</p>
<p>
Warning: as opposed to electron repulsion integrals, the
decomposition is made such that the Cholesky vectors are expanded
in a two-electron basis
\(f_{ij}(\mathbf{r}_1,\mathbf{r}_2) = \phi_i(\mathbf{r}_1) \phi_j(\mathbf{r}_2)\),
whereas in electron repulsion integrals each Cholesky vector is
expressed in a basis of a one-electron function
\(g_{ik}(\mathbf{r}_1) = \phi_i(\mathbf{r}_1) \phi_k(\mathbf{r}_1)\).
</p>
<table id="org815c311" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>1e</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">One body density matrix</td>
</tr>
<tr>
<td class="org-left"><code>1e_up</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">&uarr;-spin component of the one body density matrix</td>
</tr>
<tr>
<td class="org-left"><code>1e_dn</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num)</code></td>
<td class="org-left">&darr;-spin component of the one body density matrix</td>
</tr>
<tr>
<td class="org-left"><code>1e_transition</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[state.num, state.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, state.num, state.num)</code></td>
<td class="org-left">One-particle transition density matrices</td>
</tr>
<tr>
<td class="org-left"><code>2e</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num, mo.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo.num, mo.num)</code></td>
<td class="org-left">Two-body reduced density matrix (spin trace)</td>
</tr>
<tr>
<td class="org-left"><code>2e_upup</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num, mo.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo.num, mo.num)</code></td>
<td class="org-left">&uarr;&uarr; component of the two-body reduced density matrix</td>
</tr>
<tr>
<td class="org-left"><code>2e_dndn</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num, mo.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo.num, mo.num)</code></td>
<td class="org-left">&darr;&darr; component of the two-body reduced density matrix</td>
</tr>
<tr>
<td class="org-left"><code>2e_updn</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[mo.num, mo.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo.num, mo.num)</code></td>
<td class="org-left">&uarr;&darr; component of the two-body reduced density matrix</td>
</tr>
<tr>
<td class="org-left"><code>2e_transition</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[state.num, state.num, mo.num, mo.num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, mo.num, mo.num, state.num, state.num)</code></td>
<td class="org-left">Two-particle transition density matrices</td>
</tr>
<tr>
<td class="org-left"><code>2e_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors</td>
</tr>
<tr>
<td class="org-left"><code>2e_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[rdm.2e_cholesky_num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, rdm.2e_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the two-body RDM (spin trace)</td>
</tr>
<tr>
<td class="org-left"><code>2e_upup_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors</td>
</tr>
<tr>
<td class="org-left"><code>2e_upup_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[rdm.2e_upup_cholesky_num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, rdm.2e_upup_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the two-body RDM (&uarr;&uarr;)</td>
</tr>
<tr>
<td class="org-left"><code>2e_dndn_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors</td>
</tr>
<tr>
<td class="org-left"><code>2e_dndn_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[rdm.2e_dndn_cholesky_num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, rdm.2e_dndn_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the two-body RDM (&darr;&darr;)</td>
</tr>
<tr>
<td class="org-left"><code>2e_updn_cholesky_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Cholesky vectors</td>
</tr>
<tr>
<td class="org-left"><code>2e_updn_cholesky</code></td>
<td class="org-left"><code>float sparse</code></td>
<td class="org-left"><code>[rdm.2e_updn_cholesky_num, mo.num, mo.num]</code></td>
<td class="org-left"><code>(mo.num, mo.num, rdm.2e_updn_cholesky_num)</code></td>
<td class="org-left">Cholesky decomposition of the two-body RDM (&uarr;&darr;)</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div id="outline-container-org05dc8e5" class="outline-2">
<h2 id="org05dc8e5"><span class="section-number-2">6</span> Correlation factors</h2>
<div class="outline-text-2" id="text-6">
</div>
<div id="outline-container-orgaa6089b" class="outline-3">
<h3 id="orgaa6089b"><span class="section-number-3">6.1</span> Jastrow factor (jastrow group)</h3>
<div class="outline-text-3" id="text-6-1">
<p>
The Jastrow factor is an $N$-electron function which multiplies the CI
expansion: \(\Psi = \Phi \times \exp(J)\),
</p>
<p>
In the following, we use the notations \(r_{ij} = |\mathbf{r}_i - \mathbf{r}_j|\) and
\(R_{i\alpha} = |\mathbf{r}_i - \mathbf{R}_\alpha|\), where indices
\(i\) and \(j\) refer to electrons and \(\alpha\) to nuclei.
</p>
<p>
Parameters for multiple forms of Jastrow factors can be saved in
TREXIO files, and are described in the following sections. These
are identified by the <code>type</code> attribute. The type can be one of the
following:
</p>
<ul class="org-ul">
<li><code>CHAMP</code></li>
<li><code>Mu</code></li>
</ul>
</div>
<div id="outline-container-org4d858c9" class="outline-4">
<h4 id="org4d858c9"><span class="section-number-4">6.1.1</span> CHAMP</h4>
<div class="outline-text-4" id="text-6-1-1">
<p>
The first form of Jastrow factor is the one used in
the <a href="https://trex-coe.eu/trex-quantum-chemistry-codes/champ">CHAMP</a> program:
</p>
<p>
\[
J(\mathbf{r},\mathbf{R}) = J_{\text{eN}}(\mathbf{r},\mathbf{R}) + J_{\text{ee}}(\mathbf{r}) + J_{\text{eeN}}(\mathbf{r},\mathbf{R})
\]
</p>
<p>
\(J_{\text{eN}}\) contains electron-nucleus terms:
</p>
<p>
\[
J_{\text{eN}}(\mathbf{r},\mathbf{R}) = \sum_{i=1}^{N_\text{elec}} \sum_{\alpha=1}^{N_\text{nucl}}\left[
\frac{a_{1,\alpha}\, f_\alpha(R_{i\alpha})}{1+a_{2,\alpha}\,
f_\alpha(R_{i\alpha})} + \sum_{p=2}^{N_\text{ord}^a} a_{p+1,\alpha}\, [f_\alpha(R_{i\alpha})]^p - J_{\text{eN}}^\infty
\right]
\]
</p>
<p>
\(J_{\text{ee}}\) contains electron-electron terms:
</p>
<p>
\[
J_{\text{ee}}(\mathbf{r}) =
\sum_{i=1}^{N_\text{elec}} \sum_{j=1}^{i-1}
\left[
\frac{\frac{1}{2}\big(1 + \delta^{\uparrow\downarrow}_{ij}\big)\,b_1\, f_{\text{ee}}(r_{ij})}{1+b_2\, f_{\text{ee}}(r_{ij})} +
\sum_{p=2}^{N_\text{ord}^b} b_{p+1}\, [f_{\text{ee}}(r_{ij})]^p - J_{\text{ee},ij}^\infty
\right]
\]
</p>
<p>
\(\delta^{\uparrow\downarrow}_{ij}\) is zero when the electrons \(i\) and
\(j\) have the same spin, and one otherwise.
</p>
<p>
\(J_{\text{eeN}}\) contains electron-electron-Nucleus terms:
</p>
<p>
\[
J_{\text{eeN}}(\mathbf{r},\mathbf{R}) =
\sum_{\alpha=1}^{N_{\text{nucl}}}
\sum_{i=1}^{N_{\text{elec}}}
\sum_{j=1}^{i-1}
\sum_{p=2}^{N_{\text{ord}}}
\sum_{k=0}^{p-1}
\sum_{l=0}^{p-k-2\delta_{k,0}}
c_{lkp\alpha} \left[ g_{\text{ee}}({r}_{ij}) \right]^k \nonumber \\
\left[ \left[ g_\alpha({R}_{i\alpha}) \right]^l + \left[ g_\alpha({R}_{j\alpha}) \right]^l \right]
\left[ g_\alpha({R}_{i\,\alpha}) \,
g_\alpha({R}_{j\alpha}) \right]^{(p-k-l)/2}
\]
\(c_{lkp\alpha}\) are non-zero only when \(p-k-l\) is even.
</p>
<p>
The terms \(J_{\text{ee},ij}^\infty\) and \(J_{\text{eN}}^\infty\) are shifts to ensure that
\(J_{\text{eN}}\) and \(J_{\text{ee}}\) have an asymptotic value of zero:
</p>
<p>
\[
J_{\text{eN}}^{\infty} =
\frac{a_{1,\alpha}\, \kappa_\alpha^{-1}}{1+a_{2,\alpha}\,
\kappa_\alpha^{-1}} + \sum_{p=2}^{N_\text{ord}^a} a_{p+1,\alpha}\, \kappa_\alpha^{-p}
\]
\[
J_{\text{ee},ij}^{\infty} =
\frac{\frac{1}{2}\big(1 + \delta^{\uparrow\downarrow}_{ij}\big)\,b_1\,
\kappa_{\text{ee}}^{-1}}{1+b_2\, \kappa_{\text{ee}}^{-1}} +
\sum_{p=2}^{N_\text{ord}^b} b_{p+1}\, \kappa_{\text{ee}}^{-p}
\]
</p>
<p>
\(f\) and \(g\) are scaling function defined as
</p>
<p>
\[
f_\alpha(r) = \frac{1-e^{-\kappa_\alpha\, r}}{\kappa_\alpha} \text{ and }
g_\alpha(r) = e^{-\kappa_\alpha\, r},
\]
</p>
</div>
</div>
<div id="outline-container-org1e66881" class="outline-4">
<h4 id="org1e66881"><span class="section-number-4">6.1.2</span> Mu</h4>
<div class="outline-text-4" id="text-6-1-2">
<p>
<a href="https://aip.scitation.org/doi/10.1063/5.0044683">Mu-Jastrow</a> is based on a one-parameter correlation factor that has
been introduced in the context of transcorrelated methods. This
correlation factor imposes the electron-electron cusp, and it is
built such that the leading order in \(1/r_{12}\) of the effective
two-electron potential reproduces the long-range interaction of the
range-separated density functional theory. Its analytical
expression reads
</p>
<p>
\[
J(\mathbf{r}, \mathbf{R}) = J_{\text{eeN}}(\mathbf{r}, \mathbf{R}) +
J_{\text{eN}}(\mathbf{r}, \mathbf{R})
\].
</p>
<p>
The electron-electron cusp is incorporated in the three-body term
</p>
<p>
\[
J_\text{eeN} (\mathbf{r}, \mathbf{R}) =
\sum_{i=1}^{N_\text{elec}} \sum_{j=1}^{i-1} \, u\left(\mu, r_{ij}\right) \,
\Pi_{\alpha=1}^{N_{\text{nucl}}} \, E_\alpha({R}_{i\alpha}) \, E_\alpha({R}_{j\alpha}),
\]
</p>
<p>
where ww\(u\) is an electron-electron function
</p>
<p>
\[
u\left(\mu, r\right) = \frac{r}{2} \, \left[ 1 - \text{erf}(\mu\, r) \right] - \frac{1}{2 \, \mu \, \sqrt{\pi}} \exp \left[ -(\mu \, r)^2 \right].
\]
</p>
<p>
This electron-electron term is tuned by the parameter \(\mu\) which
controls the depth and the range of the Coulomb hole between
electrons.
</p>
<p>
An envelope function has been introduced to cancel out the Jastrow
effects between two-electrons when at least one is close to a nucleus
(to perform a frozen-core calculation). The envelope function is
given by
</p>
<p>
\[
E_\alpha(R) = 1 - \exp\left( - \gamma_{\alpha} \, R^2 \right).
\]
</p>
<p>
In particular, if the parameters \(\gamma_\alpha\) tend to zero, the
Mu-Jastrow factor becomes a two-body Jastrow factor:
</p>
<p>
\[
J_{\text{ee}}(\mathbf{r}) =
\sum_{i=1}^{N_\text{elec}} \sum_{j=1}^{i-1} \, u\left(\mu, r_{ij}\right)
\]
</p>
<p>
and for large \(\gamma_\alpha\) it becomes zero.
</p>
<p>
To increase the flexibility of the Jastrow and improve the
electron density the following electron-nucleus term is added
</p>
<p>
\[
J_{\text{eN}}(\mathbf{r},\mathbf{R}) = \sum_{i=1}^{N_\text{elec}} \sum_{\alpha=1}^{N_\text{nucl}} \,
\left[ \exp\left( a_{\alpha} R_{i \alpha}^2 \right) - 1\right].
\]
</p>
<p>
The parameter \(\mu\) is stored in the <code>ee</code> array, the parameters
\(\gamma_\alpha\) are stored in the <code>een</code> array, and the parameters
\(a_\alpha\) are stored in the <code>en</code> array.
</p>
</div>
</div>
<div id="outline-container-org0605eff" class="outline-4">
<h4 id="org0605eff"><span class="section-number-4">6.1.3</span> Table of values</h4>
<div class="outline-text-4" id="text-6-1-3">
<table id="org2b14ee2" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>type</code></td>
<td class="org-left"><code>str</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Type of Jastrow factor: <code>CHAMP</code> or <code>Mu</code></td>
</tr>
<tr>
<td class="org-left"><code>en_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Electron-nucleus parameters</td>
</tr>
<tr>
<td class="org-left"><code>ee_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Electron-electron parameters</td>
</tr>
<tr>
<td class="org-left"><code>een_num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of Electron-electron-nucleus parameters</td>
</tr>
<tr>
<td class="org-left"><code>en</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[jastrow.en_num]</code></td>
<td class="org-left"><code>(jastrow.en_num)</code></td>
<td class="org-left">Electron-nucleus parameters</td>
</tr>
<tr>
<td class="org-left"><code>ee</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[jastrow.ee_num]</code></td>
<td class="org-left"><code>(jastrow.ee_num)</code></td>
<td class="org-left">Electron-electron parameters</td>
</tr>
<tr>
<td class="org-left"><code>een</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[jastrow.een_num]</code></td>
<td class="org-left"><code>(jastrow.een_num)</code></td>
<td class="org-left">Electron-electron-nucleus parameters</td>
</tr>
<tr>
<td class="org-left"><code>en_nucleus</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[jastrow.en_num]</code></td>
<td class="org-left"><code>(jastrow.en_num)</code></td>
<td class="org-left">Nucleus relative to the eN parameter</td>
</tr>
<tr>
<td class="org-left"><code>een_nucleus</code></td>
<td class="org-left"><code>index</code></td>
<td class="org-left"><code>[jastrow.een_num]</code></td>
<td class="org-left"><code>(jastrow.een_num)</code></td>
<td class="org-left">Nucleus relative to the eeN parameter</td>
</tr>
<tr>
<td class="org-left"><code>ee_scaling</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">\(\kappa\) value in CHAMP Jastrow for electron-electron distances</td>
</tr>
<tr>
<td class="org-left"><code>en_scaling</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[nucleus.num]</code></td>
<td class="org-left"><code>(nucleus.num)</code></td>
<td class="org-left">\(\kappa\) value in CHAMP Jastrow for electron-nucleus distances</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
<div id="outline-container-org4bba6a8" class="outline-2">
<h2 id="org4bba6a8"><span class="section-number-2">7</span> Quantum Monte Carlo data (qmc group)</h2>
<div class="outline-text-2" id="text-7">
<p>
In quantum Monte Carlo calculations, the wave function is evaluated
at points of the 3N-dimensional space. Some algorithms require multiple
independent <i>walkers</i>, so it is possible to store multiple coordinates,
as well as some quantities evaluated at those points.
</p>
<p>
By convention, the electron coordinates contain first all the electrons
of $&uarr;$-spin and then all the $&darr;$-spin.
</p>
<table id="org342d8c7" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Variable</th>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Row-major Dimensions</th>
<th scope="col" class="org-left">Column-major Dimensions</th>
<th scope="col" class="org-left">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left"><code>num</code></td>
<td class="org-left"><code>dim</code></td>
<td class="org-left">&#xa0;</td>
<td class="org-left">&#xa0;</td>
<td class="org-left">Number of 3N-dimensional points</td>
</tr>
<tr>
<td class="org-left"><code>point</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[qmc.num, electron.num, 3]</code></td>
<td class="org-left"><code>(3, electron.num, qmc.num)</code></td>
<td class="org-left">3N-dimensional points</td>
</tr>
<tr>
<td class="org-left"><code>psi</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[qmc.num]</code></td>
<td class="org-left"><code>(qmc.num)</code></td>
<td class="org-left">Wave function evaluated at the points</td>
</tr>
<tr>
<td class="org-left"><code>e_loc</code></td>
<td class="org-left"><code>float</code></td>
<td class="org-left"><code>[qmc.num]</code></td>
<td class="org-left"><code>(qmc.num)</code></td>
<td class="org-left">Local energy evaluated at the points</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div id="postamble" class="status">
<p class="author">Author: TREX-CoE</p>
<p class="date">Created: 2024-06-28 Fri 08:12</p>
<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
</div>
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