cis

Configuration Interaction with Single excitations.

This program takes a reference Slater determinant of ROHF-like occupancy, and performs all single excitations on top of it. Disregarding spatial symmetry, it computes the n_states lowest eigenstates of that CI matrix. (see determinants n_states)

This program can be useful in many cases:

  1. Ground state calculation

    To be sure to have the lowest SCF solution, perform an scf (see the hartree_fock module), then a cis, save the natural orbitals (see save_natorb) and re-run an scf optimization from this MO guess.

  2. Excited states calculations

    The lowest excited states are much likely to be dominated by single-excitations. Therefore, running a cis will save the n_states lowest states within the CIS space in the EZFIO directory, which can afterwards be used as guess wave functions for a further multi-state FCI calculation if determinants read_wf is set to true before running the fci executable.

If determinants s2_eig is set to true, the CIS will only retain states having the expected \(\widehat{S^2}\) value (see determinants expected_s2). Otherwise, the CIS will take the lowest determinants n_states, whatever multiplicity they are.

Note

To discard some orbitals, use the qp_set_mo_class command 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:

  • read_wf

Calls:

  • run()

Touches:

  • fock_matrix_ao_alpha

  • fock_matrix_ao_alpha

  • mo_coef

  • level_shift

  • mo_coef

  • read_wf