Orthogonally spin‐adapted coupled‐cluster equations involving singly and doubly excited clusters. Comparison of different procedures for spin‐adaptation

Abstract

AbstractA particularly compact form of the orthogonally spin‐adapted coupled‐cluster equations involving all singly and doubly excited clusters is derived for the general case of a non‐Hartree–Fock closed‐shell reference determinant. The diagrammatic approach based on the graphical methods of spin algebras is applied. The relationship of different diagrammatic procedures for spin‐adaptation, employing both bare and spin‐adapted two‐electron interaction vertices, is discussed. A comparison with the results obtained with algebraic spin‐adaption approaches is also given.