R-matrix calculation of Rydberg states of CO

Valence and Rydberg bound states of CO are studied ab initio using the UK molecular R-matrix code and electron-CO+ scattering calculations. Results are presented for singlet and triplet Sigma(+) and Pi states of CO at R = 2.132 a(0). Various models for both the CO+ target and the full calculation are tested and the stability of the final models is demonstrated. Final calculations employ complete active space (GAS) target states, with and without further excitation, and up to 13 target states in the close-coupling expansion. The best model underestimates the ionization potential of CO by 0.1 eV and finds quantum defects systematically slightly higher than those observed, by up to 0.07. The prospects for further improvement of these calculations are discussed.