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.
This program can be useful in many cases:
* GROUND STATE CALCULATION: if even after a cis calculation, natural orbitals and then scf optimization, you are not sure to have the lowest scf solution, do the same strategy with the cisd executable to generate the natural orbitals as a guess for the scf.
* EXCITED STATES CALCULATIONS: the lowest excited states are much likely to be dominanted by single- or double-excitations.
Therefore, running a cis 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.
Also, if you specify "s2_eig" = True, the cisd will only retain states having the good value :math:`S^2` value (see expected_s2).
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 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.