25 EXCITED STATES WITH EQUATION-OF-MOTION CCSD (EOM-CCSD)

Excitation energies for singlet states can be computed using equation-of-motion (EOM) approach. For the excitation energies the EOM-CCSD method gives the same results as linear response CCSD (LR-CCSD) theory. Accurate results can only be expected for excited states dominated by single excitations. The states to be computed are specified on an EOM input card, which is a subcommand of CCSD. The following input forms are possible

EOM, *state1, state2, state3, ...*

Computes the given states. Each state is specified in the form *number.sym*,
e.g., 5.3 means the fifth state in symmetry 3. Note that state 1.1 corresponds
to the ground state CCSD wavefunction and is ignored if given.

EOM, *, , ...*

computes the first states in symmetry *sym1*, in *sym2* etc.

EOM, *, , ...*

computes states through in symmetry *sym1*.

The different forms can be combined, e.g.,

EOM, , , ,

computes states 1-3 in symmetry 1, the second excited state in symmetry 2, and the second through fifth excited states in symmetry 3. Note that state 1.1 is the ground-state CCSD wavefunction.

By default, an error exit will result if the CCSD did not converge and a subsequent EOM calculation is attempted. The error exit can be avoided using the NOCHECK option on the CCSD command (see also CCSD(T)).