Reciprocal adjustment of approximate coupled cluster and configuration interaction approaches

摘要

The linear version of the externally corrected coupled cluster method with singles and doubles (ecLCCSD), the recently proposed coupled cluster corrections to the multireference configuration interaction (ccMRCI) energies, and the so-called self-consistent, size-consistent [(SC)(2)] approaches, which are designed to correct for the dynamic correlation effects and the size inconsistency of the MRCI energies, are analyzed and compared using several illustrative examples, including the dissociation of a triple-zeta (TZ) model of the N-2 molecule. It is emphasized that the exponential cluster ansatz for the wave function is the basis of all these approaches, and appropriate cluster analysis of the MRCI wave function is the key step for both ecLCCSD and ccMRCI. The contributions from the orthogonal complement of the MRCI space, which can be generated by relying on such a cluster analysis, are responsible for a substantial part of the missing correlation energy. The ecLCCSD approach seems to represent a particularly attractive alternative to other highly accurate methods for the calculation of the ground-state energy in the presence of quasidegeneracy, both due to its efficiency and affordability. It may in fact be regarded as a simple alternative to the iterative reduced multireference (RMR) CCSD method. (C) 2000 John Wiley & Sons, Inc. [References: 57]