Nonempirical calculations on the electronic spectrum of the molecular ion C2 -

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Abstract
Ab initio SCF and CI calculations are reported for the molecular ion C2 - using an atomic basis set of contracted Gaussian lobe functions from a work of Whitten and employing a moderate configuration interaction which includes only valence excited configurations. Reasonable agreement is obtained with the spectroscopic constants re and omega e of the upper and the lower state of the Herzberg-Lagerqvist band system (2 Sigma u +;2 Sigma g +), while the transition energy T0 is overestimated by approximately 30%. Two hitherto unobserved electronic states are predicted: a low-lying 2 Pi u state (re=1.32 AA, omega e=1700 cm-1, Te=4800 cm-1) and a 4 Sigma u + state about 0.6 eV higher than the 2 Sigma u + state. The accuracy of the SCF results is tested by comparative calculations of other 13-electron molecules (CN, CO+), which are better known from experiment. The CI results are compared with previous calculations on C2.