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

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The 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.

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International Journal of Quantum Chemistry