Direct selected configuration interaction using a hole-particle formalism

Abstract The outlines of a direct selected configuration interaction algorithm are given. The selected determinants φ I are identified as products of α and β spins multiple excitations from the reference determinant φ 0 , in a hole-particle formalism. This formulation is compact and in the action of the αα (or ββ) part of H on the current vector avoids consideration of the excitation processes, saving significant computation time. A few examples illustrate the performance of the method, taken from NH 3 for which full-CI is known, or a more realistic problem, NiCO 2 , 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.

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