A fully ab initio potential curve of near-spectroscopic quality for OH − ion: importance of connected quadruple excitations and scalar relativistic effects

Abstract

A benchmark study has been carried out on the ground-state potential curve of the hydroxyl anion, OH −, including detailed calibration of both the 1-particle and n-particle basis sets. The CCSD(T) basis set limit overestimates ω e by about 10 cm −1, which is only remedied by inclusion of connected quadruple excitations in the coupled cluster expansion — or, equivalently, the inclusion of the 2π orbitals in the active space of a multireference calculation. Upon inclusion of scalar relativistic effects (−3 cm −1 on ω e), a potential curve of spectroscopic quality (sub-cm −1 accuracy) is obtained. Our best computed EA(OH), 1.828 eV, agrees to three decimal places with the best available experimental value. Our best computed dissociation energies, D 0(OH −)=4.7796 eV and D 0(OH)=4.4124 eV, suggest that the experimental D 0(OH)=4.392 eV may possibly be about 0.02 eV too low.