A Fast Implementation of Perfect Pairing and Imperfect Pairing Using the Resolution of the Identity Approximation.

Abstract

We present an efficient implementation of the perfect pairing and imperfect pairing coupled-cluster methods, as well as their nuclear gradients, using the resolution of the identity approximation to calculate two-electron integrals. The perfect pairing and imperfect pairing equations may be solved rapidly, making integral evaluation the bottleneck step. The method's efficiency is demonstrated for a series of linear alkanes, for which we show significant speed-ups (of approximately a factor of 10) with negligible error. We also apply the imperfect pairing method to a model of a recently synthesized stable singlet biradicaloid based on a planar Ge-N-Ge-N ring, confirming its biradical character, which appears to be remarkably high.