MRCI study of potential energy curves, spectroscopic and molecular properties of the CO+ cation

The potential energy curves (PECs) of nine low-lying electronic states (X2Σ+, A2Π, B2Σ+, C2Δ, D2Π, a4Σ+, 14Δ, b4Π and 14Σ−) of the CO+ cation have been studied using an ab initio quantum chemical method. The calculations have been performed employing the complete active space self-consistent field (CASSCF) method followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with a correlation-consistent aug-cc-pV5Z basis set. To improve the quality of the PECs, core–valence correlation and relativistic corrections are included. Relativistic corrections are included using the third-order Douglas–Kroll Hamiltonian approximation. Core–valence correlation corrections are included using a cc-pCVQZ basis set. Relativistic corrections are included at the level of the cc-pV5Z basis set. In order to obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size-extensivity errors by means of the Davidson modification (MRCI+Q). Analyses show that the effects on the spectroscopic parameters by the core–valence correlation correction, relativistic correction and Davidson modification are obvious, whereas the effect on the spectroscopic parameters by the nonadiabatic correction can be neglected. For the X2Σ+, A2Π, B2Σ+ and C2Δ electronic states, six main isotopologues (12C16O+, 12C17O+, 12C18O+, 13C16O+, 13C17O+ and 13C18O+) have been studied, since their experimental spectroscopic parameters and molecular constants are relatively abundant. For the D2Π, a4Σ+, 14Δ, 14Σ− and b4Π electronic states, just the most naturally abundant isotopologue, 12C16O+, is investigated. These results have been compared in detail with those reported in the literature. Excellent agreement has been found between the present results and the experimentally determined ones. The molecular constants of the C2Δ and the spectroscopic parameters of the D2Π, a4Σ+, 14Δ, 14Σ− and b4Π electronic states determined by the MRCI+Q/aug-cc-pV5Z+CV+DK calculations are expected to be reliable predicted results.