Truncated version of the reduced multireference coupled-cluster method with perturbation selection of higher than pair clusters

Abstract

A general implementation of a perturbatively truncated version of the reduced multireference (RMR) coupled-cluster method with singles and doubles (CCSD) [Peris, G. et al. J Chem Phys 1999, 110, 11708]—representing the MR-CISD-based version of the so-called externally corrected CCSD—is described and tested on several molecular systems involving the dissociation of a single bond of different character (HF and F2) as well as of a triple bond (N2). The possible pitfalls of a straightforward application of a perturbative selection scheme that is based on the first-order wave function coefficients are discussed, and different selection schemes are examined. It is shown that in order to obtain a stable and reliable scheme, it is necessary to exclude from the selection process all singles (and preferably all singles and doubles) relative to the leading Hartree–Fock reference configuration and retain them in the MR CISD wave function used as the external source of higher than pair connected clusters. The benefits of using perturbatively truncated RMR CCSD based on a large active space versus a nontruncated one, relying on a small model space, are also pointed out. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 743–756, 2000