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===


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 HartreeFock (singlereference 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.




