cisd

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 determinants n_states).

This program can be useful in many cases:

• Ground state calculation: if even after a cis() calculation, natural orbitals (see save_natorb()) and then scf() optimization, you are not sure to have the lowest scf solution, do the same strategy with the cisd() executable instead of the cis() exectuable 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 cisd() will save the “n_states” lowest states within the CISD space in the EZFIO directory, 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 determinants read_wf). Also, if you specify “s2_eig” = True, the cisd will only retain states having the good value \(S^2\) value (see determinants expected_s2 and 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 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

  • “del” orbitals which will be never occupied

Needs:

n_states

read_wf

Calls:

run()

Touches:

fock_matrix_ao_alpha fock_matrix_ao_alpha

mo_coef level_shift

mo_coef read_wf