Excitation energies of H2O, N2 and C2 in full configuration interaction and coupled cluster theory

Singlet excitation energies of H2O, N2 and C2 have been calculated in full configuration interaction (FCI) and in the coupled cluster model hierarchy CCS, CC2, CCSD and CC3. Excitation energies are improved at each level in the coupled cluster hierarchy, with a decrease in the error compared to FCI of about a factor of three at each level. This decrease is in accordance with the fact that the excitations in CCS, CC2, CCSD and CC3 are correct through higher and higher order in the fluctuation potential, and that more and more completer cluster treatments are used. Non-iterative triples corrections to the CCSD excitation energies are compared with the iterative triples models CC3 and FCI. The CCSDR(3) approach recovers the major part of the correlation improvement obtained in the CC3 model.