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https://github.com/LCPQ/quantum_package
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169 lines
4.8 KiB
ReStructuredText
169 lines
4.8 KiB
ReStructuredText
.. _cisd:
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.. program:: cisd
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.. default-role:: option
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====
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cisd
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====
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This module contains a CI of single and double excitations.
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The user point of view
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----------------------
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The :command:`cisd` program performs the CI of the ROHF-like + all single and double excitations on top of it.
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This program can be very useful to :
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* **Ground state calculations**: generate a guess for the ground state wave function if one is not sure that the :c:func:`scf` program gave the lowest SCF solution. In combination with :c:func:`save_natorb` it can produce new |MOs| in order to reperform an :c:func:`scf` optimization.
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* **Excited states calculations**: generate guess for all the :option:`determinants n_states` wave functions, that will be used by the :c:func:`fci` program.
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The main keywords/options to be used are:
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* :option:`determinants n_states` : number of states to consider for the |cisd| calculation
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* :option:`determinants s2_eig` : force all states to have the desired value of :math:`S^2`
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* :option:`determinants expected_s2` : desired value of :math:`S^2`
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The programmer point of view
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----------------------------
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This module have been built by setting the following rules:
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* The only generator determinant is the Hartree-Fock (single-reference method)
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* All generated determinants are included in the wave function (no perturbative
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selection)
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These rules are set in the ``H_apply.irp.f`` file.
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EZFIO parameters
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----------------
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.. option:: energy
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Variational |CISD| energy
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Subroutines / functions
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-----------------------
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.. c:function:: cisd
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.. code:: text
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subroutine cisd
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File: :file:`cisd.irp.f`
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Configuration Interaction with Single and Double excitations.
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This program takes a reference Slater determinant of ROHF-like occupancy,
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and performs all single and double excitations on top of it, disregarding spatial symmetry and compute the "n_states" lowest eigenstates of that CI matrix (see :option:`determinants n_states`).
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This program can be useful in many cases:
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* GROUND STATE CALCULATION: if even after a :c:func:`cis` calculation, natural orbitals (see :c:func:`save_natorb`) and then :c:func:`scf` optimization, you are not sure to have the lowest scf solution, do the same strategy with the :c:func:`cisd` executable instead of the :c:func:`cis` exectuable to generate the natural orbitals as a guess for the :c:func:`scf`.
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* EXCITED STATES CALCULATIONS: the lowest excited states are much likely to be dominanted by single- or double-excitations. Therefore, running a :c:func:`cisd` will save the "n_states" lowest states within the CISD space in the EZFIO folder, which can afterward be used as guess wave functions for a further multi-state fci calculation if you specify "read_wf" = True before running the fci executable (see :option:`determinants read_wf`). Also, if you specify "s2_eig" = True, the cisd will only retain states having the good value :math:`S^2` value (see :option:`determinants expected_s2` and :option:`determinants s2_eig`). If "s2_eig" = False, it will take the lowest n_states, whatever multiplicity they are.
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Note: if you would like to discard some orbitals, use :ref:`qp_set_mo_class` to specify:
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* "core" orbitals which will be always doubly occupied
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* "act" orbitals where an electron can be either excited from or to
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* "del" orbitals which will be never occupied
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.. c:function:: h_apply_cisd
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.. code:: text
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subroutine H_apply_cisd()
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File: :file:`h_apply.irp.f_shell_8`
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Calls H_apply on the |HF| determinant and selects all connected single and double excitations (of the same symmetry). Auto-generated by the ``generate_h_apply`` script.
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.. c:function:: h_apply_cisd_diexc
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.. code:: text
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subroutine H_apply_cisd_diexc(key_in, key_prev, hole_1,particl_1, hole_2, particl_2, fock_diag_tmp, i_generator, iproc_in )
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File: :file:`h_apply.irp.f_shell_8`
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.. c:function:: h_apply_cisd_diexcorg
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.. code:: text
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subroutine H_apply_cisd_diexcOrg(key_in,key_mask,hole_1,particl_1,hole_2, particl_2, fock_diag_tmp, i_generator, iproc_in )
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File: :file:`h_apply.irp.f_shell_8`
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Generate all double excitations of key_in using the bit masks of holes and particles. Assume N_int is already provided.
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.. c:function:: h_apply_cisd_diexcp
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.. code:: text
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subroutine H_apply_cisd_diexcP(key_in, fs1, fh1, particl_1, fs2, fh2, particl_2, fock_diag_tmp, i_generator, iproc_in )
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File: :file:`h_apply.irp.f_shell_8`
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.. c:function:: h_apply_cisd_monoexc
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.. code:: text
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subroutine H_apply_cisd_monoexc(key_in, hole_1,particl_1,fock_diag_tmp,i_generator,iproc_in )
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File: :file:`h_apply.irp.f_shell_8`
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Generate all single excitations of key_in using the bit masks of holes and particles. Assume N_int is already provided.
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