PNNL

Abstract

Several new variants of the locally-renormalized coupled-cluster (CC) approaches that account for the effect of triples (LR-CCSD(T)) have been formulated an implemented for arbitrary reference states using the TENSOR CONTRACTION ENGINE functionality, enabling an efficient parallel execution of the program. Deeply rooted in the recently derived Numerator-Denominator Connected (NDC) expansion for the ground-state energy [K. Kowalski, P. Piecuch, J. Chem. Phys. 122, 07107 (2005), LR-CCSD(T) approximations use, in analogy to the completely-renormalized CCSD(T) (CR-CCSD(T) approach, the three-body moments in constructing the non-iterative correction s to the energies obtained in CC calculations with singles and doubles. In contrast to the CR-CCSD(T) method, the LR-CCSD(T) approach employs local denominators which assure the additive separability of the energies in the non-interacting system limit when the localized basis set is employed in the CCSD and LR-CCSD(T) calculations. As clearly demonstrated on several challenging examples, including breaking the bonds of the F2, N2, and CN molecules, the LR-CCSD(T) approaches are capable of providing a highly accurate description of the entire potential energy surface (PES) while maintaining the characteristic N7 scaling of the ubiquitous CCSD(T) approach. Moreover, as illustrated numerically for the ozone molecule, the LR-CCSD(T) approaches yield highly competitive values for a number of equilibrium properties including bond lengths, angles and harmonic frequencies.