A comparative study of core-extensive and core-valence-extensive coupled-cluster theories for energy differences: excitation energies

Abstract

We have investigated the relative computational efficacies of the open-shell coupled-cluster (OSCC) theory and the CC-based linear response theory (CC-LRT) for computing excitation energies directly. It is emphasized that the core-valence-extensive nature of OSCC and the core-extensive nature of CC-LRT stem essentially from the differences in the cluster structures of the wave operators. Results obtained from CH⁺, H₂O and N₂ indicate that both methods perform well for low-lying excited states. The model space used in our OSCC method consists of h-p determinants, and consequently, the excited states dominated by the 2h-2p determinants are poorly described. In contrast, these states are well described in CC-LRT, since the working space in this case has both h-p and 2h-2p determinants. We comment on the relative ease of implementation of the two methods.