# 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:

> 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.

> 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`