Triplet excitation energies in full configuration interaction and coupled-cluster theory

Triplet excitation energies have been calculated for Ne, H2O, HF, BH, N-2, and C-2 using the full configuration interaction (FCI) model and the couple d-cluster model hierarchy CCS, CC2, CCSD, CC3, and CCSDT, where CCS, CCSD, and CCSDT are the standard coupled-cluster models where singles, doubles an d triples are successively added and where CC2 and CC3 are approximations t o the CCSD and CCSDT models where approximations are introduced in the high est amplitude equations. Comparing the coupled-cluster excitation energies with the FCI results shows that the excitation energies are improved at eac h level in the hierarchy up to CC3. The CC3 and CCSDT excitation energies h ave similar accuracy for the single excitation dominated excitation energie s, whereas the double excitation dominated excitation energies are signific antly improved also from CC3 to CCSDT. Singlet excitation energies have als o been calculated for HF using the coupled-cluster hierarchy up to CCSDT. T riplet and singlet excitation energies with similar single excitation weigh ts have about the same accuracy. (C) 2001 American Institute of Physics.