Coupled cluster method
Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Abstract The ability of various approximate coupled cluster (CC) methods to provide accurate first-order one-electron properties calculated as expectation values is theoretically analysed and computationally examined for BH and CO. For actual calculations the infinite number of terms of the expectation value expansion (〈O〉=〈φ¦exp (T +)O exp (T)¦φ〉c) was truncated so that T 1 T 2, T 3, and (1/2) T 2T2 clusters were retained on both sides of O. The role of individual clusters is carefully discussed. Inclusion of T 1, is unavoidable, but if triples are essential in the energy evaluation, they may play an even more important role in the property expansion, as shown in the case of CO. It is shown that the CC wave function, which is exact to second order, effectively satisfies the Hellmann-Feynman theorem.
Type of Medium: