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

Abstract

A reformulation of the perturbative triples correction to coupled cluster singles and doubles (CCSD) based on the numerical Laplace transform of the energy denominator is presented. Rearranged equations reduce the O(N7) canonical scaling to O(N6), 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 orbital representations. Furthermore, substituting canonical by generalized CCSD natural orbitals, the Laplace ansatz exhibits scaling close to O(N5), while retaining accuracy and providing crossover with respect to canonical triples for small size systems. A developing atomic orbital formulation is also introduced.