===
cis
===
This module contains a |CIS| program.
The user point of view
----------------------
The :ref:`cis` program performs the CI to obtain the ROHF reference + all
single 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 :ref:`scf` program gave the lowest |SCF|
solution. In combination with :ref:`save_natorb` it can produce new |MOs| in
order to reperform an :ref:`scf` optimization.
* **Excited states calculations**: generate guesses for all the
:option:`determinants n_states` wave functions, that will be used by the
:ref:`fci` program.
The main keywords/options to be used are:
* :option:`determinants n_states`: number of states to consider for the |CIS| calculation
* :option:`determinants s2_eig`: force all states to have the desired value of |S^2|
* :option:`determinants expected_s2`: desired value of |S^2|
The programmer's point of view
------------------------------
This module was built by setting the following rules:
* The only generator determinant is the Hartree-Fock (single-reference method)
* All generated singly excited determinants are included in the wave function (no perturbative
selection)
These rules are set in the ``H_apply.irp.f`` file.
Anthony Scemama
16565bbda4