A b i n i t i o study of perturbations between the X 1Σ+g and b 3Σ−g states of the C2 molecule

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Abstract
We report ab initio calculations of the energies and wavefunctions for the X 1Σ+g and b 3Σ−g states of C2 at several levels of accuracy. The best calculations, using a double‐zeta basis of Gaussian‐lobe functions and including dσ and dπ functions and extensive CI, give good agreement with experimental potential curves. These wavefunctions are used to compute spin–orbit coupling between the two states. The results agree well with the matrix elements obtained from the perturbations observed by Ballik and Ramsay. Some implications of our results for the theory of intersystem crossing are discussed.