An ab initio diabatic study of rovibronic spectra of CN

An ab initio study of the rovibronic spectra for the cyano radical (CN) based on a diabatic representation is presented. This work considers 17 electronic states, 59 dipole moment curves, 88 spin-orbit coupling curves, and 30 electronic angular momentum coupling curves, which are obtained using the internally contracted multireference configuration interaction method including the Davidson correction (icMRCI + Q) with the aug-cc-pwCV5Z-DK basis set. The diabatic transformations are performed based on a property-based diabatization method to remove the avoided crossings for the D²Π-H²Π and b⁴Π-2⁴Π pairs. Ab initio potential energy curves of the X²Σ⁺, B²Σ⁺, E²Σ⁺, A²Π, D²Π, H²Π, F²Δ and J²Δ electronic states are shifted to match the experimental electronic excitation energies and the equilibrium internuclear distances. The coupled nuclear motion Schrödinger equations are then solved to obtain the rovibronic spectra of CN for wavenumbers from 0 to 80 000 cm^-1. At wavenumbers of 0-30 000 cm^-1, our absorption cross sections agree well with available theoretical data. For wavenumbers above 30 000 cm^-1, our cross sections are larger than previous data in view of the fact that the transitions involving high-lying electronic states are considered. This work provides an overall prediction of the rovibronic spectrum of CN. Our results are suitable for temperatures below 8000 K and could be useful for the investigations of planetary exploration.