Scaling reduction of the perturbative triples correction (T) to coupled cluster theory via Laplace transform formalism

A reformulation of the perturbative triples correction to coupled cluster s ingles and doubles (CCSD) based on the numerical Laplace transform of the e nergy denominator is presented. Rearranged equations reduce the O(N-7) cano nical scaling to O(N-6), where N is a size measure of the electronic system . Two to three quadrature points is adequate for chemical predictions. The Laplace ansatz permits simple, noniterative expressions in noncanonical orb ital representations. Furthermore, substituting canonical by generalized CC SD natural orbitals, the Laplace ansatz exhibits scaling close to O(N-5), w hile retaining accuracy and providing crossover with respect to canonical t riples for small size systems. A developing atomic orbital formulation is a lso introduced. (C) 2000 American Institute of Physics. [S0021-9606(00)3014 7-7].