11.3 Calculation of Excitation Energies

At the (conventional) CCSD level also electronic excitation energies can be computed. For this the data group $excitations has to be added (the same keyword as for CC2 applies). The implementation is currently restricted to vertical excitation energies (no transition moments or properties available) and in the closed–shell case to singlet excited states.

Note that for single-excitation dominated transitions CCSD is — like CC2 — only correct through second-order and is not necessarily more accurate than CC2. It is, however, for double excitations still correct through first-order, while CC2 describes double excitations only in a zero-order approximation. Therefore, CCSD results are more robust with respect to double excitation contributions to transitions and are thus usefull to check if CC2 is suitable for a certain problem.

Since release V7.9 excitation energies for singlet excitations for a closed-shell reference wavefunction are also available for the iterative approximate triples model CC3 and the perturbative triples corrections CCSDR(T) and CCSDR(3).