Theoretical characterization of the low-lying excited states of the CuCl molecule

The character of the low-lying excited states of diatomic CuCl is studied primarily by means of the complete active space self-consistent field (CASSCF), method and a second order perturbation approach with the CASSCF wave function as reference state [complete active space perturbation theory to second order (CASPT2)]. For comparison, the lower levels of the spectra of the Cu+ ion are also analyzed. A first order treatment of the scalar relativistic effects, the mass–velocity and Darwin terms, is included in the calculations. The importance of spin–orbit interactions is investigated by comparing our nonrelativistic valence shell CI (VCI) and relativistic results obtained with our four-component program suite MOLFDIR. The six lowest excited states of the CuCl molecule, which are related to the Cu+(3d94s1)Cl−(3s23p6) ionic configuration, are assigned. The assignments agree with earlier theoretical work. Where they can be compared, the calculated spectroscopic constants are in good agreement with the experimental data.