SIZE-CONSISTENT SINGLE-REFERENCE METHODS FOR ELECTRONIC CORRELATION -A UNIFIED FORMULATION THROUGH INTERMEDIATE HAMILTONIAN THEORY

Using the intermediate hamiltonian theory as a unique conceptual frame and the technique of CI matrix dressing, a wide series of single-refe rence methods for the treatment of the ground state correlation are re viewed, compared, and sometimes improved. These methods range from ind ependent excitation approximation (the very next step beyond MP2) to c oupled cluster, going through the so-called electron pair approximatio ns and the (SC)(CI)-C-2 formalism, A hierarchy of these methods can be established according to two criteria: 1. The physical effects incorp orated in the model space, the choice of which is flexible. 2. The qua lity of the evaluation of the coefficients of the external space deter minants. This evaluation, which remains based on a single reference ex pansion of the wave function, may simply ensure the size consistency o r incorporate the linked contributions from the outer space. These for mulations in terms of diagonalizations of dressed CI matrices avoid co nvergence problems, but their main advantage is their flexibility, sin ce they apply to multi-reference SDCI spaces as well as to SDCI spaces . The use of a common frame allows one to propose consistent combinati ons of methods of various costs for the treatment of various parts of the correlation energy.