Electron Correlation in Small Molecules: Grafting CI onto CC

Abstract

Among the post-Hartree-Fock methods, those based on the coupled cluster (CC) ansatz for the electronic wave function proved to be extremely valuable in quantum chemical computations of the molecular electronic structure, being capable of attaining chemical accuracy for many molecular properties of interest. While the widely exploited single reference (SR) singles and doubles CC method (CCSD) is remarkably efficient in handling dynamic correlation, a proper account of nondynamic correlation, which becomes essential in the presence of the quasidegeneracy, requires multireference (MR) formalism. In view of the complexity and computational demands of the available MR CC methods, it is highly desirable to design SR CCSD-type approaches that are capable of accommodating both types of correlation effects. One avenue to achieve this goal is offered by the so-called externally corrected (ec) CCSD methods, which exploit some independent source of higher than pair clusters — whose importance rises with the increasing quasidegeneracy — to correct the standard CCSD equations. In view of the complementarity of SR CC and MR configuration interaction (CI) methods in their ability to describe the dynamic and nondynamic correlation effects, a particularly suitable and affordable external source proved to be an MR CISD wave function, based on a small active or model space, leading to the so-called reduced multireference (RMR) CCSD approach. Following a brief outline of the origins and of the status quo of the ecCCSD and RMR CCSD methodologies, their performance is illustrated by a few examples, and their potential and relationship with other approaches is discussed.