SELF-CONSISTENT INTERMEDIATE HAMILTONIANS - A COUPLED-CLUSTER TYPE FORMULATION OF THE SINGLES AND DOUBLES CONFIGURATION-INTERACTION MATRIX DRESSING

This paper presents a new self-consistent dressing of a singles and do ubles configuration interaction matrix which insures size-consistency, separability into closed-shell subsystems if localized molecular orbi tals (MOs) are used, and which includes all fourth order corrections. This method yields, among several schemes, a reformulation of the coup led cluster method, including fully the cluster operators of single an d double excitations, and partially those of the triples (Bartlett's a lgorithm named CCSDT-1a). Further improvement can be easily included b y adding exclusion principle violating corrections. Since it leads to a matrix diagonalization, the method behaves correctly in case of near degeneracies between the reference determinant and some doubles. Due to its flexibility this formulation offers the possibility of consiste nt combination with less expensive treatments for the study of very la rge systems. (C) 1995 American Institute of Physics.