Assessment of the orbital-optimized coupled-electron pair theory for thermochemistry and kinetics: improving on CCSD and CEPA(1).

An assessment of the orbital-optimized coupled-electron pair theory [or simply "optimized CEPA(0)," OCEPA(0)] [Bozkaya and Sherrill, J. Chem. Phys. 2013, 139, 054104] for thermochemistry and kinetics is presented. The OCEPA(0) method is applied to closed- and open-shell reaction energies, barrier heights, and radical stabilization energies (RSEs). The performance of OCEPA(0) is compared with those of the MP2, CEPA(0), OCEPA(0), CEPA(1), coupled-cluster singles and doubles (CCSD), and CCSD(T) methods [at complete basis set limits employing cc-pVTZ and cc-pVQZ basis sets]. For the most of the test sets, the OCEPA(0) method performs better than CEPA(0), CEPA(1), and CCSD, and provides accurate results. Especially, for open-shell reaction energies and barrier heights, the OCEPA(0)-CEPA(1) and OCEPA(0)-CCSD differences become obvious. Similarly, for barrier heights and RSEs, the OCEPA(0) method improves on CEPA(0) by 1.6 and 2.3 kcal mol(-1) . Our results demonstrate that the CEPA(0) method dramatically fails when the reference wave function suffers from the spin-contamination problem. Conversely, the OCEPA(0) method can annihilate spin-contamination in the unrestricted-Hartree-Fock initial guess orbitals and can yield stable solutions. For overall evaluation, we conclude that the OCEPA(0) method is quite helpful not only for problematic open-shell systems and transition-states but also for closed-shell molecules. Hence, one may prefer OCEPA(0) over CEPA(0), CEPA(1), and CCSD as an O(N6) method, where N is the number of basis functions, for thermochemistry and kinetics. As discussed previously, the cost of the OCEPA(0) method is as much as of CCSD and CEPA(1) for energy computations. However, for analytic gradient computations, the OCEPA(0) method is two times less expensive than CCSD and CEPA(1). Further, the stationary properties of the OCEPA(0) method making it promising for excited state properties via linear response theory.