Fully ab initio spectroscopic quality potential curve for the 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 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. (C) 2001 Elsevier Science B.V. All rights reserved.