Nondynamic Correlation and Coupled-Cluster Methods

Abstract

The coupled‐cluster (CC) methods that are based on the exponential Ansatz for the wave operator proved to be extremely valuable in quantum chemical computations of the molecular electronic structure and represent nowadays most accurate and often used post‐Hartree‐Fock approaches that are capable to attain chemical accuracy for many molecular properties of interest. Nonetheless, while the widely exploited single‐reference (SR) CC approaches, such as the CCSD method accounting for one‐ and two‐body cluster amplitudes or its CCSD(T) version perturbatively corrected for three‐body clusters—all available in numerous software packages—are remarkably efficient in handling of dynamical correlation effects, their performance rapidly deteriorates in the presence of quasidegeneracy of the reference configuration when the nondynamic correlations become important. A possible, computationally affordable, remedy for this failure is the so‐called reduced multireference (RMR) CCSD method, as well as its triple‐corrected version RMR CCSD(T). These methods exploit the complementarity of the configuration interaction (CI) and CC approaches and represent the topic of this communication.