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 ¹Σ_g⁺ state of the π_u(4) configuration (of 2p orbitals), the ³Π_u and ¹Π_u states of the ${\pi }_u{\text{(3)σ}}_g\text{(1)}$ configuration, and the ${}^3{\Sigma }_g^{-}{,}^1{\Delta }_g{,}^1{\Sigma }_g^{+}$ states of the ${\pi }_u{\text{(2)σ}}_g\text{(2)}$ configuration. The calculated energies of the ¹Σ_g⁺ and ¹Π_u electronic states agree well with experiment. Finally, a critical analysis is given of the coefficients of the expansion in covalent and ionic structures.