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.