Theoretical study of the perturbation parameters in the a 3Π and A 1Π states of CO

Completely ab initio calculations of the perturbation parameters of the a 3Π and A 1Π states of CO have been carried out and compared with the recent experimental results of Field et al. Use has been made of a new program for calculating off-diagonal matrix elements of the B L · S and spin-orbit coupling operators. By including configuration interaction functions built from Hartree-Fock orbitals optimized for each state, good agreement has been obtained between calculated and experimental constants in most cases. Our results show that the perturbations have a strong dependence on R and that they are different for different pairs of states. The single configuration approximation gives a relative error up to 40% and the constancy of perturbation values found by Field et al. cannot be attributed to dependence on a single, constant parameter.