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

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 ⁺, H ₂O and N ₂ 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.