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 singly excited states. 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)).