RESTRICTED OPEN-SHELL HARTREE-FOCK-BASED MANY-BODY PERTURBATION-THEORY - THEORY AND APPLICATION OF ENERGY AND GRADIENT CALCULATIONS

A new approach for many-body perturbation theory (MBPT) built upon a restricted open-shell Hartree-Fock (ROHF) reference function is presented. ROHF-MBPT is shown to give much improved results compared to unrestricted Hartree-Fock (UHF) MBPT in cases where there is large spin contamination of the UHF reference function, and to converge much more rapidly to the infinite-order coupled-cluster result. Equations for analytical gradients at the MBPT(2) level are described and implemented. ROHF-MBPT and restricted open-shell Hartree-Fock single- and double-excitation coupled cluster (ROHF-CCSD) applications are presented for several difficult cases. These include the structure and electron affinity of the CN radical; structure, binding energy, and vibrational frequencies of Li3; the structure and vibrational frequencies for the unobserved FCS molecule; and the multiplet structure of the Ni atom.