This module contains a CI of single and double excitations.
The user point of view
----------------------
The :command:`cisd` program performs the CI of the ROHF-like + all single and double excitations on top of it.
This program can be very useful to :
***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.
***Excited states calculations**: generate guess for all the :option:`determinants n_states` wave functions, that will be used by the :c:func:`fci` program.
The main keywords/options to be used are:
*:option:`determinants n_states` : number of states to consider for the |cisd| calculation
*:option:`determinants s2_eig` : force all states to have the desired value of :math:`S^2`
*:option:`determinants expected_s2` : desired value of :math:`S^2`
The programmer point of view
----------------------------
This module have been built by setting the following rules:
Configuration Interaction with Single and Double excitations.
This program takes a reference Slater determinant of ROHF-like occupancy,
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`).
This program can be useful in many cases:
* 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`.
* 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.
Note: if you would like to discard some orbitals, use :ref:`qp_set_mo_class` to specify:
* "core" orbitals which will be always doubly occupied
* "act" orbitals where an electron can be either excited from or to
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.