USING, FULL-CI ALGORITHMS IN BETHE-GOLDSTONE-TYPE EXPANSIONS OF THE CORRELATION-ENERGY

In the early 1960s, Nesbet proposed to develop correlation energy in t erms of two-, three-, four-, etc., electron contributions. This expans ion was, in principle, applicable to a large number of electrons witho ut a size-extensivity error. The now available full-CI algorithms may be used to obtain those expansions in terms of either occupied spin-or bitals or, more efficiently, in terms of sets of occupied or virtual m olecular orbitals. Tests on the NH, molecule with a DZP basis-set prob lem show the slow convergence of this approach. (C) 1997 John Wiley & Sons, Inc.