A comparative study of core-extensive and core—valence-extensive coupled-cluster theories for energy differences: Excitation energies
✍ Scribed by Saroj Kr Mukhopadhyay; Rajat Chaudhuri; Debasis Mukhopadhyay Jr.; Debashis Mukherjee
- Publisher
- Elsevier Science
- Year
- 1990
- Tongue
- English
- Weight
- 546 KB
- Volume
- 173
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
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+, Hz0 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.