22.3 Excited state calculations

There are two possibilities to perform excited state calculations:

1) One can calculate each state separately. This is done using the card

STATE,1,root

where root is the desired root (i.e., 2 for the first excited state). In this case the Fock operator used in the zeroth-order hamiltonian is computed using the density for the given state.

2) Alternatively, two or more states can be computed simultaneously, using

STATE, n[,root1, root2, ..., rootn]

where n is the number of states to be computed. The default is to compute the lowest n roots. Optionally, this default can be modified by specifying the desired roots rooti as shown. One should note that this does not correspond to the multi-state CASPT2 presented in [Chem. Phys. Lett. 288, 299 (1998)].

In the case that several states are computed simultaneously, the fock operator employed in the zeroth-order Hamiltonian is computed from a state-averaged density matrix, and the zeroth-order hamiltonians for all states are constructed from the same fock operator. By default, equal weights for all states are used. This default can be modified using the WEIGHT directive

WEIGHT,w1, w2,...,wn.

If a REFSTATE card is given (see section 17.3.5), the state-averaged fock operator is made for all reference states, and the WEIGHT card refers to the corresponding states.