Locally renormalized coupled-cluster equations for singly and doubly excited clusters†

Abstract

The Numerator-Denominator Connected (NDC) Expansion for the Coupled-Cluster (CC) method [K. Kowalski and P. Piecuch, J. Chem. Phys. 122, 074107 (2005)], is used to construct a new set of stationary conditions for approximate coupled-cluster approaches. Several CC approximations based on models involving singles and doubles (CCSD) are developed and discussed in the context of ground-state applications. The resulting locally-renormalized CCSD (LR-CCSD) equations are shown to regularize the expressions for the cluster amplitudes in the challenging situations that occur when the orbital energy differences approach zero. Affordable schemes for handling the local denominators (all-holes-Jn coupling), that naturally appear in locally renormalized formalisms, are also discussed. †Dedicated to Professor Andrzej J. Sadlej on the occasion of his 65th birthday.