A note on the direct calculation of excitation energies by quasi-degenerate MBPT and coupled-cluster theory

✍ Scribed by Dhiman Sinha; Saroj Mukhopadhyay; Debashis Mukherjee

• Publisher: Elsevier Science
• Year: 1986
• Tongue: English
• Weight: 533 KB
• Volume: 129
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(86)80361-x

✦ Synopsis

For excitation energy calculations using quasidegenerate MBPT or coupled-cluster (CC) theory, the hitherto chosen strategy has been to have particle-hole (p-h) determinants forming the model space and to use standard formalisms origlnally developed for complete model spaces. In view of our recent analysis, the p-h determinants constitute an incomplete model space for which intermediate normalization (IN) of the wave-operator n is not appropriate if a linked formulation is deshed. The theoretical status of earlier applications which had used formulae appropriate to IN, yet ignored disconnected diagrams, is analyzed. It is shown that if only excitation energies, rather than the full 52, are desired, then no theoretical error is made. The situation is, however, fortuitous in that for mp-mh model spaces with m > 1, a similar procedure using IN would necessarily generate disconnected diagrams.