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@ -1,13 +1,33 @@
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%% This BibTeX bibliography file was created using BibDesk.
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%% http://bibdesk.sourceforge.net/
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%% Created for Pierre-Francois Loos at 2020-08-18 10:07:35 +0200
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%% Created for Pierre-Francois Loos at 2020-08-18 15:08:15 +0200
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%% Saved with string encoding Unicode (UTF-8)
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@article{Kutzelnigg_1985,
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Author = {W. Kutzelnigg},
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Date-Added = {2020-08-18 15:05:16 +0200},
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Date-Modified = {2020-08-18 15:05:51 +0200},
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Journal = {Theor. Chim. Acta},
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Pages = {445},
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Title = {r12-Dependent terms in the wave function as closed sums of partial wave amplitudes for large l},
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Volume = {68},
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Year = {1985}}
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@article{Kutzelnigg_1991,
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Author = {W. Kutzelnigg and W. Klopper},
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Date-Added = {2020-08-18 15:05:16 +0200},
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Date-Modified = {2020-08-18 15:05:24 +0200},
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Journal = {J. Chem. Phys.},
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Pages = {1985},
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Title = {Wave functions with terms linear in the interelectronic coordinates to take care of the correlation cusp. I. General theory},
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Volume = {94},
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Year = {1991}}
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@article{Pack_1966,
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Author = {{R. T. Pack and W. Byers-Brown}},
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Date-Added = {2020-08-18 09:51:18 +0200},
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@ -943,8 +963,9 @@
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Year = {2015},
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Bdsk-Url-1 = {https://doi.org/10.1063/1.4907920}}
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@article{GinPraFerAssSavTou-JCP-18,
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@article{Giner_2018,
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Author = {Emmanuel Giner and Barth\'elemy Pradines and Anthony Fert\'e and Roland Assaraf and Andreas Savin and Julien Toulouse},
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Date-Modified = {2020-08-18 15:08:15 +0200},
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Journal = {J. Chem. Phys.},
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Pages = {194301},
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Title = {Curing basis-set convergence of wave-function theory using density-functional theory: A systematically improvable approach},
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@ -120,7 +120,7 @@ Another route to solve the Schr\"odinger equation is density-functional theory (
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Present-day DFT calculations are almost exclusively done within the so-called Kohn-Sham (KS) formalism, \cite{Kohn_1965} which
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transfers the complexity of the many-body problem to the universal and yet unknown exchange-correlation (xc) functional thanks to a judicious mapping between a non-interacting reference system and its interacting analog which both have exactly the same one-electron density.
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KS-DFT \cite{Hohenberg_1964,Kohn_1965} is now the workhorse of electronic structure calculations for atoms, molecules and solids thanks to its very favorable accuracy/cost ratio. \cite{ParrBook}
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As compared to WFT, DFT has the indisputable advantage of converging much faster with respect to the size of the basis set. \cite{FraMusLupTou-JCP-15,Loos_2019d,Giner_2020}
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As compared to WFT, DFT has the indisputable advantage of converging much faster with respect to the size of the basis set. \cite{FraMusLupTou-JCP-15,Giner_2018,Loos_2019d,Giner_2020}
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However, unlike WFT where many-body perturbation theory provides a precious tool to go toward the exact wave function, there is no systematic way to improve approximate xc functionals toward the exact functional.
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Therefore, one faces, in practice, the unsettling choice of the \emph{approximate} xc functional. \cite{Becke_2014}
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Moreover, because of the approximate nature of the xc functional, although the resolution of the KS equations is variational, the resulting KS energy does not have such property.
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@ -397,7 +397,7 @@ single- or multi-determinant wave function $\Psi^{(0)}$ which can
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be obtained in many different ways depending on the system that one considers.
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One of the particularity of the present work is that we
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use the CIPSI algorithm to perform approximate FCI calculations
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with the RS-DFT Hamiltonian $\hat{H}^\mu$. \cite{GinPraFerAssSavTou-JCP-18}
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with the RS-DFT Hamiltonian $\hat{H}^\mu$. \cite{Giner_2018}
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This provides a multi-determinant trial wave function $\Psi^{\mu}$ that one can ``feed'' to DMC.
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In the outer (macro-iteration) loop (red), at the $k$th iteration, a CIPSI selection is performed
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to obtain $\Psi^{\mu\,(k)}$ with the RS-DFT Hamiltonian $\hat{H}^{\mu\,(k)}$
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@ -656,14 +656,14 @@ This is yet another key result of the present study.
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\begin{figure*}
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\includegraphics[width=\columnwidth]{density-mu.pdf}
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\includegraphics[width=\columnwidth]{on-top-mu.pdf}
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\caption{\toto{One-electron density $n(\br)$ (left) and on-top pair
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\caption{One-electron density $n(\br)$ (left) and on-top pair
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density $n_2(\br,\br)$ (right) along the \ce{O-H} axis of \ce{H2O}
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as a function of $\mu$ for $\Psi^\mu$, and $\Psi^J$ (dashed
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curve).
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The integrated on-top pair density $\expval{P}$ is
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given in the legend.
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For all trial wave functions, the CI expansion consists of the 200 most important
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determinants of the FCI expansion obtained with the VDZ-BFD basis (see Sec.~\ref{sec:rsdft-j} for more details).}}
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determinants of the FCI expansion obtained with the VDZ-BFD basis (see Sec.~\ref{sec:rsdft-j} for more details).}
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\label{fig:densities}
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\end{figure*}
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%%% %%% %%% %%%
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@ -671,9 +671,9 @@ This is yet another key result of the present study.
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In order to refine the comparison between $\Psi^\mu$ and $\Psi^J$, we
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report several quantities related to the one- and two-body densities of
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$\Psi^J$ and $\Psi^\mu$ with different values of $\mu$. First, we
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report in the legend of Fig~\ref{fig:densities} the integrated on-top pair density
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report in the legend of the right panel of Fig~\ref{fig:densities} the integrated on-top pair density
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\begin{equation}
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\expval{ P } = \int d\br \,\,n_2(\br,\br)
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\expval{ P } = \int d\br \,\,n_2(\br,\br),
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\end{equation}
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where $n_2(\br_1,\br_2)$ is the two-body density [normalized to $\Nelec(\Nelec-1)$]
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obtained for both $\Psi^\mu$ and $\Psi^J$.
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@ -691,13 +691,13 @@ are much more important than that of the one-body density, the latter
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being essentially unchanged between $\mu=0$ and $\mu=\infty$ while the
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former can vary by about 10$\%$ in some regions.
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%TODO TOTO
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\toto{In the high-density region of the \ce{O-H} bond, the value of the on-top
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In the high-density region of the \ce{O-H} bond, the value of the on-top
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pair density obtained from $\Psi^J$ is superimposed with
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$\Psi^{\mu=0.5}$, and at a large distance the on-top pair density of $\Psi^J$ is
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the closest to $\mu=\infty$. The integrated on-top pair density
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obtained with $\Psi^J$ is $\expval{P}=1.404$, between the values obtained with
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obtained with $\Psi^J$ is $\expval{P}=1.404$, which nestles between the values obtained at
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$\mu=0.5$ and $\mu=1$~bohr$^{-1}$, consistently with the FN-DMC energies
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and the overlap curve depicted in Fig.~\ref{fig:overlap}.}
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and the overlap curve depicted in Fig.~\ref{fig:overlap}.
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These data suggest that the wave functions $\Psi^{0.5 \le \mu \le 1}$ and $\Psi^J$ are close,
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and therefore that the operators that produced these wave functions (\ie, $H^\mu[n]$ and $e^{-J}He^J$) contain similar physics.
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@ -748,18 +748,18 @@ As a conclusion of the first part of this study, we can highlight the following
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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Atomization energies are challenging for post-HF methods
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because their calculation requires a perfect balance in the
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because their calculation requires a subtle balance in the
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description of atoms and molecules. The mainstream one-electron basis sets employed in molecular
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calculations are atom-centered, so they are, by construction, better adapted to
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atoms than molecules and atomization energies usually tend to be
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underestimated by variational methods.
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In the context of FN-DMC calculations, the nodal surface is imposed by
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the trial wavefunction which is expanded in an atom-centered basis
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set, so we expect the fixed-node error to be also intimately related to
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the trial wavefunction which is expanded in the very same atom-centered basis
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set. Thus, we expect the fixed-node error to be also intimately related to
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the basis set incompleteness error.
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Increasing the size of the basis set improves the description of
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the density and of the electron correlation, but also reduces the
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imbalance in the quality of the description of the atoms and the
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imbalance in the description of atoms and
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molecule, leading to more accurate atomization energies.
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%============================
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@ -768,19 +768,21 @@ molecule, leading to more accurate atomization energies.
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An extremely important feature required to get accurate
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atomization energies is size-consistency (or strict separability),
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since the numbers of correlated electron pairs in the isolated atoms
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are different from those of the molecules.
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The energy computed within DFT is size-consistent, and
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as it is a mean-field method the convergence to the CBS limit
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is relatively fast. Hence, DFT methods are very well adapted to
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since the numbers of correlated electron pairs in the molecule and its isolated atoms
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are different.
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The energies computed within DFT are size-consistent, and
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\titou{because it is a mean-field method the convergence to the CBS limit
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is relatively fast}. \cite{FraMusLupTou-JCP-15}
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Hence, DFT methods are very well adapted to
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the calculation of atomization energies, especially with small basis
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sets. But going to the CBS limit will converge to biased atomization
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energies because of the use of approximate density functionals.
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sets. \cite{Giner_2018,Loos_2019d,Giner_2020}
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\titou{But going to the CBS limit will converge to biased atomization
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energies because of the use of approximate density functionals.}
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On the other hand, FCI is also size-consistent, but the convergence of
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the FCI energies to the CBS limit is much slower because of the
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Likewise, FCI is also size-consistent, but the convergence of
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the FCI energies towards the CBS limit is much slower because of the
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description of short-range electron correlation using atom-centered
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functions. But ultimately the exact energy will be reached.
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functions. \cite{Kutzelnigg_1985,Kutzelnigg_1991} But ultimately the exact energy will be reached.
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In the context of SCI calculations, when the variational energy is
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extrapolated to the FCI energy \cite{Holmes_2017} there is no
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@ -805,7 +807,7 @@ one-electron, two-electron and one-nucleus-two-electron terms.
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The problematic part is the two-electron term, whose simplest form can
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be expressed as in Eq.~\eqref{eq:jast-ee}.
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The parameter
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$a$ is determined by cusp conditions, and $b$ is obtained by energy
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$a$ is determined by the electron-electron cusp condition, \cite{Kato_1957,Pack_1966} and $b$ is obtained by energy
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or variance minimization.\cite{Coldwell_1977,Umrigar_2005}
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One can easily see that this parameterization of the two-body
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interaction is not size-consistent: the dissociation of a
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@ -816,11 +818,11 @@ size-consistency error on a PES using this ans\"atz for $J_\text{ee}$,
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one needs to impose that the parameters of $J_\text{ee}$ are fixed:
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$b_A = b_B = b_{\ce{AB}}$.
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When pseudopotentials are used in a QMC calculation, it is common
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When pseudopotentials are used in a QMC calculation, it is of common
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practice to localize the non-local part of the pseudopotential on the
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complete wave function (determinantal component and Jastrow).
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complete trial wave function $\Phi$.
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If the wave function is not size-consistent,
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so will be the locality approximation. Within, the DLA,\cite{Zen_2019} the Jastrow factor is
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so will be the locality approximation. Within the DLA,\cite{Zen_2019} the Jastrow factor is
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removed from the wave function on which the pseudopotential is localized.
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The great advantage of this approximation is that the FN-DMC energy
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only depends on the parameters of the determinantal component. Using a
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@ -834,9 +836,9 @@ Ref.~\onlinecite{Scemama_2015}).
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%%% TABLE III %%%
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\begin{table}
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\caption{FN-DMC energies (in hartree) using the VDZ-BFD basis set
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and pseudopotential of the fluorine atom and the dissociated fluorine
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dimer, and size-consistency error. }
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\caption{FN-DMC energy (in hartree) using the VDZ-BFD basis set and the srPBE functional
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of the fluorine atom and the dissociated \ce{F2} molecule for various $\mu$ values.
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The size-consistency error is also reported.}
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\label{tab:size-cons}
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\begin{ruledtabular}
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\begin{tabular}{cccc}
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@ -863,7 +865,7 @@ of this system is equal to twice the energy of the fluorine atom.
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The data in Table~\ref{tab:size-cons} shows that it is indeed the
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case, so we can conclude that the proposed scheme provides
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size-consistent FN-DMC energies for all values of $\mu$ (within
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$2\times$ statistical error bars).
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twice the statistical error bars).
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%============================
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@ -893,8 +895,8 @@ impacted by this spurious effect, as opposed to FCI.
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%%% TABLE IV %%%
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\begin{table}
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\caption{FN-DMC energies (in hartree) of the triplet carbon atom (VDZ-BFD) with
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different values of $m_s$.}
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\caption{FN-DMC energy (in hartree) for various $\mu$ values of the triplet carbon atom with
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different values of $m_s$ computed with the VDZ-BFD basis set and the srPBE functional.}
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\label{tab:spin}
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\begin{ruledtabular}
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\begin{tabular}{cccc}
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@ -937,15 +939,15 @@ noticeable with $\mu=5$~bohr$^{-1}$.
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%%% FIG 6 %%%
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\begin{squeezetable}
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\begin{table*}
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\caption{Mean absolute errors (MAE), mean signed errors (MSE) and
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standard deviations (RMSD) obtained with various methods and
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\caption{Mean absolute errors (MAE), mean signed errors (MSE), and
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root mean square errors (RMSE) obtained with various methods and
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basis sets.}
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\label{tab:mad}
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\begin{ruledtabular}
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\begin{tabular}{ll ddd ddd ddd}
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& & \mc{3}{c}{VDZ-BFD} & \mc{3}{c}{VTZ-BFD} & \mc{3}{c}{VQZ-BFD} \\
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\cline{3-5} \cline{6-8} \cline{9-11}
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Method & $\mu$ & \tabc{MAE} & \tabc{MSE} & \tabc{RMSD} & \tabc{MAE} & \tabc{MSE} & \tabc{RMSD} & \tabc{MAE} & \tabc{MSE} & \tabc{RMSD} \\
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Method & $\mu$ & \tabc{MAE} & \tabc{MSE} & \tabc{RMSE} & \tabc{MAE} & \tabc{MSE} & \tabc{RMSE} & \tabc{MAE} & \tabc{MSE} & \tabc{RMSE} \\
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\hline
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PBE & 0 & 5.02 & -3.70 & 6.04 & 4.57 & 1.00 & 5.32 & 5.31 & 0.79 & 6.27 \\
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BLYP & 0 & 9.53 & -9.21 & 7.91 & 5.58 & -4.44 & 5.80 & 5.86 & -4.47 & 6.43 \\
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@ -976,20 +978,21 @@ DMC@ & 0 & 4.61(34) & -3.62(34) & 5.30(09) & 3.52(19) & -
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\end{squeezetable}
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%%% %%% %%% %%%
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The 55 molecules of the benchmark for the Gaussian-1
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theory\cite{Pople_1989,Curtiss_1990} were chosen to test the
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The atomization energies of the 55 molecules of the Gaussian-1
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theory\cite{Pople_1989,Curtiss_1990} were chosen as a benchmark set to test the
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performance of the RS-DFT-CIPSI trial wave functions in the context of
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energy differences. Calculations were made in the double-, triple-
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energy differences. \titou{REFERENCES??}
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Calculations were made in the double-, triple-
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and quadruple-$\zeta$ basis sets with different values of $\mu$, and using
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NOs from a preliminary CIPSI calculation \titou{as a starting point}.
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NOs from a preliminary CIPSI calculation as a starting point (see Fig.~\ref{fig:algo}).
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For comparison, we have computed the energies of all the atoms and
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molecules at the KS-DFT level with various semi-local and hybrid density functionals [PBE, BLYP, PBE0, and B3LYP], and at
|
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the CCSD(T) level. Table~\ref{tab:mad} gives the corresponding mean
|
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absolute errors (MAE), mean signed errors (MSE) and standard
|
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deviations (RMSD). For FCI (RS-DFT-CIPSI, $\mu=\infty$) we have
|
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absolute errors (MAE), mean signed errors (MSE), and root mean square errors (RMSE).
|
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For FCI (RS-DFT-CIPSI, $\mu=\infty$) we have
|
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provided the extrapolated values at $\EPT \to 0$, and the error bars
|
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correspond to the difference between the energies \titou{computed with a
|
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two-point and with a three-point linear extrapolation}. \cite{Loos_2018a,Loos_2019,Loos_2020b,Loos_2020c}
|
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correspond to the difference between the extrapolated energies computed with a
|
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two-point and a three-point linear extrapolation. \cite{Loos_2018a,Loos_2019,Loos_2020b,Loos_2020c}
|
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In this benchmark, the great majority of the systems are weakly correlated and are then well
|
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described by a single determinant. Therefore, the atomization energies
|
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@ -998,7 +1001,7 @@ the basis set is small. The introduction of exact exchange (B3LYP and
|
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PBE0) make the results more sensitive to the basis set, and reduce the
|
||||
accuracy. Thanks to the single-reference character of these systems,
|
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the CCSD(T) energy is an excellent estimate of the FCI energy, as
|
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shown by the very good agreement of the MAE, MSE and RMSD of CCSDT(T)
|
||||
shown by the very good agreement of the MAE, MSE and RMSE of CCSDT(T)
|
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and FCI energies.
|
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The imbalance of the quality of description of molecules compared
|
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to atoms is exhibited by a very negative value of the MSE for
|
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@ -1022,9 +1025,9 @@ trial wave function was estimated for each system by searching for the
|
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minimum of the spline interpolation curve of the FN-DMC energy as a
|
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function of $\mu$.
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This corresponds the line of Table~\ref{tab:mad} labelled as ``Opt.''
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\titou{The optimal $\mu$ value for each system is reported in the \SI.}
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The optimal $\mu$ value for each system is reported in the \SI.
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Using the optimal value of $\mu$ clearly improves the
|
||||
MAE, the MSE an the RMSD compared to the FCI wave function. This
|
||||
MAE, the MSE an the RMSE compared to the FCI wave function. This
|
||||
result is in line with the common knowledge that re-optimizing
|
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the determinantal component of the trial wave function in the presence
|
||||
of electron correlation reduces the errors due to the basis set incompleteness.
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@ -1059,11 +1062,11 @@ $\mu=0.5$~bohr$^{-1}$ with the quadruple-$\zeta$ basis set.
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%%% %%% %%% %%%
|
||||
|
||||
Searching for the optimal value of $\mu$ may be too costly and time consuming, so we have
|
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computed the MAD, MSE and RMSD for fixed values of $\mu$.
|
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computed the MAD, MSE and RMSE for fixed values of $\mu$.
|
||||
As illustrated in Fig.~\ref{fig:g2-dmc} and Table \ref{tab:mad},
|
||||
the best choice for a fixed value of $\mu$ is
|
||||
0.5~bohr$^{-1}$ for all three basis sets. It is the value for which
|
||||
the MAE [$3.74(35)$, $2.46(18)$, and $2.06(35)$ kcal/mol] and RMSD [$4.03(23)$,
|
||||
the MAE [$3.74(35)$, $2.46(18)$, and $2.06(35)$ kcal/mol] and RMSE [$4.03(23)$,
|
||||
$3.02(06)$, and $2.74(13)$ kcal/mol] are minimal. Note that these values
|
||||
are even lower than those obtained with the optimal value of
|
||||
$\mu$. Although the FN-DMC energies are higher, the numbers show that
|
||||
|
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