Potential curves for inner-shell states of CO calculated at multiconfigurational self-consistent field level

A general strategy to calculate potential curves at multiconfigurational self-consistent field (MCSCF) level for inner-shell states is reported in this paper. Convergence is commonly very tough for inner-shell states, especially at this level of calculation, due to the problem of variational collapse of the inner-shell wave function to the ground or to a low-lying excited state. The present method allows to avoid this drawback by a sequence of constrained optimization in the orbital mixing step. The specific states studied are that resulting from transitions X (1)Σ(+) → (C 1s(-1) π(∗)) (1,3)Π of CO. Accurate values are achieved for transition energies and vibrational splittings. A comparison is made with other approach, i.e., inner-shell CI based on a MCSCF wave function optimized for ground or low-lying excited states. This last approach is shown to fail in describing the whole potential curve.