Potential energy curves for the (1)Sigma(+) and (1,3)Pi states of CO

Ab initio potential energy curves of CO are calculated to address a number of problems remaining in the interpretation of the experimental VUV absorption spectra. The calculations are of the type SCF MRSD-CI. We employed the aug-cc-pVQZ basis set for both carbon and oxygen, augmented with twelve diffuse functions, of s-, p-, and d-type, located on both atomic centers. We focus on the energy region 85,000 cm(-1) < E < 110,000 cm(-1) characterized by strong interactions between Rydberg and valence states. In this work we deal specifically with the (1)Sigma(+), (1)Pi, and (3)Pi states lying in this region. Some of the relevant findings are as follows: The minimum of the C'(1)Sigma(+) valence state is found 1920 cm(-1) above the value inferred from an extrapolation of experimental data. A new (1)Pi valence state, labeled E', is found to perturb strongly the (X(2)Sigma(+))3ppi E(1)Pi Rydberg state. The electrostatic perturbation of the (X(2)Sigma(+))3ppi c(3)Pi Rydberg state by the k(3)Pi valence state is confirmed. The energy position of the (A(2)Pi)3ssigma (3)Pi state, the triplet companion of the W(1)Pi Rydberg state, is predicted.