# 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.