Noniterative inclusion of the triply and quadruply excited clusters: The locally renormalized perspective

Noniterative inclusion of the higher-order clusters has been a subject of intensive studies aimed at developing a well balanced description of individual many-body contributions for entire ground-state potential energy surfaces. In traditional approaches, the connected quadruples are estimated directly based on perturbative arguments, which leads to excellent agreement with full CI results near the equilibrium geometry and increasingly worse energies for larger internuclear stretches. As a possible improvement to this situation, two techniques are considered as especially promising: perturbative approaches based on the similarity transformed Hamiltonians and renormalization schemes both in global and local formulations. Following the latter strategy we adopted the recently introduced numerator-denominator connected expansion (NDC) [K. Kowalski, P. Piecuch, J. Chem. Phys. 122 (2005) 074107] as an effective tool for designing new forms of noniterative corrections accounting for the joint effect of triples and quadruples. The performance of the ensuing locally renormalized CCSD(TQ) approaches (LR-CCSD(TQ)) is illustrated on several examples that require either going beyond the triples approximation or describing very subtle effects encountered in van der Waals complexes. Comparisons with other noniterative approaches are also made and some issues regarding the size-extensivity of the locally renormalized methods are addressed.