Low Excited States in C2

In this paper the energies of the low electronic states of the C2 molecule are calculated, using a semi-empirical approach. In the calculation, the two-center core integrals, and the one-center Coulomb integral, are empirical parameters which are obtained by fitting of the Morse potential curves and from atomic data, respectively. The standard ASMO-LCAO state wave functions are expanded in covalent, single ionic, and double ionic components, which are constructed so as to obey the Wigner-Wittmer dissociation rules at infinite internuclear distances. The contributions for each component of the expanded state wave functions are obtained by the variational method. The states considered are 1Σg+ state of the πu(4) configuration (of 2p orbitals), the 3Πu and 1Πu states of the πu(3)σg(1) configuration, and the 3Σg−,1Δg,1Σg+ states of the πu(2)σg(2) configuration. The calculated energies of the 1Σg+ and 1Πu electronic states agree well with experiment. Finally, a critical analysis is given of the coefficients of the expansion in covalent and ionic structures.