21.2 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
as described in section 21.3.

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 20.2.11), the state-averaged
fock operator is made for all reference states, and the `WEIGHT`
card refers to the corresponding states.

molpro@molpro.net 2019-03-24