MRCI study on spectroscopic and molecular properties of several low-lying electronic states of the CN radical

The potential energy curves (PECs) of eight low-lying electronic states (X2Σ+, A2Π, B2Σ+, a4Σ+, D2Π, E2Σ+, 12Σ- and F2Δ) of the CN radical have been studied using the ab initio quantum chemical method. The calculations have been performed using the complete active space self-consistent field (CASSCF) method followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with the correlation-consistent basis sets of Dunning and co-workers. The effects on the PECs by the core-valence correlation and relativistic corrections are taken into account. The way to consider the relativistic correction is to use the second-order Douglas-Kroll Hamiltonian approximation. The core-valence correlation correction calculations are performed with the cc-pCVQZ basis set. The relativistic correction is carried out at the level of 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). The PECs are extrapolated to the complete basis set (CBS) limit by the total-energy extrapolation scheme. With these PECs, the spectroscopic parameters (Te, Re, ωe, ωexe, ωeye, Be, αe and γe) are determined and compared with those reported in the literature. Finally, with the PECs obtained by the MRCI+Q/CV+DK+Q5 calculations, the complete vibrational states are computed for the eight electronic states by solving the ro-vibrational Schroedinger equation for the non-rotating radical, and the vibrational levels and inertial rotation and centrifugal distortion constants of the first 11 vibrational states are reported, which agree favorably with the available experimental data. The spectroscopic parameters of 12Σ- and F2Δ electronic states obtained by the MRCI+Q/CV+DK+Q5 calculations should be good predictions for future laboratory experiments.