Biorthogonal method of moments of coupled‐cluster equations: Alternative derivation, further considerations, and application to a model magnetic system

Abstract

AbstractThe energy expansion defining the biorthogonal method of moments of coupled‐cluster equations (MMCC) [Piecuch and Włoch, J Chem Phys, 2005, 123, 224105 and Piecuch et al., Chem Phys Lett 2006, 418, 467], which leads to the size extensive completely renormalized (CR) coupled‐cluster (CC) approach with singles, doubles, and noniterative triples employing the left eigenstates of the similarity‐transformed Hamiltonian, termed CR‐CC(2,3), is overviewed and rederived. The rederivation of the biorthogonal MMCC expansion presented in this work is based on a direct resummation and subsequent elimination of the many‐body components of the exponential wave operator of CC theory that appear at individual moment contributions in the original MMCC energy expansion [Kowalski and Piecuch, J Chem Phys, 2000, 113, 18; Kowalski and Piecuch, J Chem Phys 2001, 115, 2966], enabling one to understand why the CR‐CC(2,3) method using the biorthogonal MMCC theory is more accurate than the earlier CR‐CCSD(T) approach. The superiority of the CR‐CC(2,3) method over the CR‐CCSD(T) and other previously developed single‐reference CC methods with a noniterative treatment of triply excited clusters, including the widely used CCSD(T) approach and the triples corrections defining the CCSD(2) schemes, is illustrated by examining the singlet–triplet gap of the (HFH)− magnetic system in which two paramagnetic centers are linked via a polarizable diamagnetic bridge. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008