A computational study of the non-adiabatic coupling among low-lying doublet states of the CN radical

Abstract

First principle electronic structure calculations on low-lying doublet states of the CN radical have been performed using the multi-reference configuration interaction method and large all-electron basis sets. Beside the adiabatic interatomic potentials, the non-adiabatic spin-orbit and L-uncoupling electronic matrix elements were evaluated for the mutually perturbed (2)B2Σ+∼(1–4)2Π and (1)A2Π∼(1–3)2Σ+, (1)2Σ−, (1,2)2Δ pairs of states. The radial coupling matrix element was calculated between the X2Σ+ and B2Σ+ states. The reliability of the derived A∼X and A∼B matrix elements was confirmed by comparison with their empirical counterparts. Both local and regular perturbations in the A∼B∼X complex were found to depend mostly on spin-orbit and electronic-rotational interactions between the lowest A2Π, and B2Σ+ states. Excited states, in all their diversity, have little effect on the fine structure of the A∼B complex, except for the qA Λ-doubling function. The ab initio coupling functions can be crucial to providing a global deperturbation treatment for the A, B and X states on a spectroscopic level of accuracy.