A fully {it ab initio} potential curve of near-spectroscopic quality for the OH^- anion: importance of connected quadruple excitations and scalar relativistic effects

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 $\omega_e$ 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 $2\pi$ orbitals in the active space of a multireference calculation. Upon inclusion of scalar relativistic effects (-3 cm^{-1} on $\omega_e$), 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.