Approximate Coupled Cluster Methods: Combined Reduced Multireference and Almost-Linear Coupled Cluster Methods with Singles and Doubles

Abstract

Abstract The recently introduced version of the externally corrected coupled cluster method with singles and doubles (CCSD), exploiting a small active space multireference (MR) configuration interaction with singles and doubles (CISD) wave function as a source of the three and four body connected cluster amplitudes, which is referred to as the reduced MR (RMR) CCSD method, and the so-called almost-linear (AL) CCSD method based on the split–amplitude strategy, are combined to achieve a considerable saving of the computational effort, while yielding almost exact RMR CCSD results. The performance of the suggested approach is exemplified on several models involving four and eight hydrogen atoms in various geometric configurations that enable the quasidegeneracy effects to be varied from the completely degenerate limit to the nondegenerate case, as well as on the symmetrically stretched double zeta model of the water molecule and of the fluorine molecule. In all cases the suggested approach provides an excellent approximation to the full RMR CCSD energies, which in turn faithfully account for the nondynamical correlation effects involved.