Electronic Structure and Bonding of Cobalt Monoxide, CoO, and Its Ions CoO⁺ and CoO^–: An Ab Initio Study

We present a systematic and high-level ab initio study of CoO and its ions, CoO⁺ and CoO^–. Employing variational multireference (MRCI) and single-reference coupled–cluster methods combined with basis sets of quintuple quality, we have calculated 50, 31, and 7 bound states for CoO, CoO⁺, and CoO^–, respectively. For all these states, complete potential energy curves have been constructed at the MRCI level of theory, whereas for a few low-lying states core subvalence and scalar relativistic effects have been taken into account. We report energetics, spectroscopic parameters, dipole moments, and spin–orbit coupling constants. The ground states of CoO, CoO⁺, and CoO^– are X⁴Δ, X⁵Δ, and X⁵Δ, respectively, the latter established for the first time. The CoO is quite ionic with a Co to O Mulliken charge transfer of ∼0.6 electrons and a dipole moment μ­(X⁴Δ) = 4.5 ± 0.1 D. The overall agreement between theory and experiment is good, but there are also important deviations. Despite the seeming simplicity of these diatomic species, reliable results can only be obtained at a high level of theory