A Critical Assessment of Multireference-Fock Space CCSD and Perturbative Third-Order Triples Approximations for Photoelectron Spectra and Quasidegenerate Potential Energy Surfaces

Equations for the Fock space coupled cluster method, including all single, double, and triple excitations (FSCCSDT) for ionization potentials [(0,1) sector], are presented in both operator and spin orbital form. Two approximations to the full FSCCSDT equations are described, one being the simplest perturbative inclusion of triple excitation effects, FSCCSD+T(3), and a second that indirectly incorporates certain higher-order effects, FSCCSD+T ^* (3). The performance of these approximations, along with FSCCSD, has been studied in comparison with single reference coupled cluster (CC) methods and with experiment for a variety of ionization potential (IP) and potential energy surface (PES) problems. (CH ₂ , CH ₂ NH, CH ₂ PH, O ₂ , H ₂ CO, N ₂ , ketene, and diazomethane for IPs and CO ₂ , NO ₃ , and C ₃ ⁺ for PES.) For ionization potentials, the FSCCD and FSCCSD+T ^* (3) methods are generally found to perform comparably to single reference CC with perturbative inclusion of triple excitations overall, while for the more demanding application to potential energy surfaces, FSCC methods with the current perturbative triple corrections do not appear to match the high-quality single reference CC results. But because of the speed and ease of use compared to single reference CC methods, FSCC methods, including triple excitation effects, are sufficiently accurate to be extremely useful tools when used with care. Unlike most single reference methods, Fock space CC methods are also particularly suited to "final state" studies of quasidegenerate potential energy surfaces. Several examples of this type "CO ₂ , N0 ₃ , and C ⁺ ₃ "are analyzed.