DIRECT SELECTED CONFIGURATION-INTERACTION USING A HOLE PARTICLE FORMALISM

The outlines of a direct selected configuration interaction algorithm are given. The selected determinants PHI(I) are identified as products of alpha and beta-spins multiple excitations from the reference determinant PHI-0, in a hole-particle formalism. This formulation is compact and in the action of the alpha-alpha (or beta-beta) part of H on the current vector avoids consideration of the excitation proccsses. saving significant computation time. A few examples illustrate the performance of the method, taken from NH3 for which full-CI is known, or a more realistic problem, NiCO2, involving a large number of active MOs (61) and electrons (26). The possible uses of the algorithm in approximate CI methods like CIPSI are discussed.