Accurate Electron Affinities from the Extended Koopmans' Theorem Based on Orbital-Optimized Methods.

The extended Koopmans' theorem (EKT) provides a systematic way to compute electron affinities (EAs) from any level of theory. Although, it is widely applied to ionization potentials, the EKT approach has not been extensively applied to computations of electron affinities. We present the first benchmarking study to investigate the performances of the EKT methods for predictions of EAs. We assess the performances of the EKT approaches based on orbital-optimized methods [Bozkaya, U. J. Chem. Phys. 2013, 139, 154105], such as the orbital-optimized third-order Møller-Plesset perturbation theory and the orbital-optimized coupled-electron pair theory [OCEPA(0)], and their standard counterparts for EAs of the selected atoms, closed- and open-shell molecules. Especially, results of the OCEPA(0) method (with the aug-cc-pVQZ basis set) for EAs of the considered atoms and molecules are very promising, the corresponding mean absolute errors are 0.14 and 0.17 eV, respectively.