Fock space multireference coupled-cluster theory for general single determinant reference functions

Abstract

The technique of Fock space multireference coupled-cluster (FSMRCC) theory is applied for the first time to problems involving a high-spin open-shell ground state. Explicit spin–orbital equations applicable to any single determinant reference state are presented and some computational aspects of FSMRCC are discussed. The method is illustrated by two applications in which calculations are limited to single and double excitation operators (FSMRCCSD). First, several basis sets and choices of open-shell reference function are used to calculate selected ionization potentials of O2. The FSMRCCSD results obtained with a large generally contracted basis set are uniformly within 0.1 eV of experiment. In addition, FSMRCCSD is applied to a study of symmetry breaking in the 3A2 state of CO2, a classic multireference problem. The force constant for asymmetric distortion is shown to be predicted correctly as positive, unlike ordinary single-reference CCSD which predicts a double-minimum potential. The results of this paper suggest that the open-shell reference FSMRCC approach has wide applicability for the solution of chemical problems, particularly when significant nondynamic electron correlation effects are present.