Excited states

It is possible to run excited states calculations with the quantum package. To do this, set determinants n_states to the number of requested states. The selection criterion will be the maximum of the selection criteria for each state. If the Davidson diagonalization has difficulties to converge, increase the davidson n_states_diag value.

When computing multiple states, it is good to have the determinants s2_eig flag true. This will force the Davidson algorithm to choose only vectors with a value of \(\widehat{S^2}\) equal to determinants expected_s2. Otherwise, different spin states will come out in the diagonalization.

The Quantum Package doesn’t take account of the symmetry. Due to numerical noise, excited states of different symmetries may enter in the calculation. Note that it is possible to make state-average calculation of states with different symmetries and/or different spin multiplicities.

To include excited states of all possible symmetries, a simple trick is to run a preliminary multi-state CIS calculation using the cis program, and then running the selected FCI restarting from the CIS states, setting determinants read_wf to true.

Usually, it is good practice to use state-averaged natural MOs so that all states have MOs of comparable quality. This allows for a faster convergence of excitation energies.

See also

The documentation of the scf, cis and fci programs.