N (1A g), T (3B1u), and V (1B1u) states of vertical ethylene

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Abstract
We present the most exhaustive theoretical studies to date of the nature of the lowest (ππ*) singlet state of vertical C2H4. Basis sets as large as double zeta plus polarization plus diffuse functions were used in conjunction with large scale configuration interaction, including as many as 7064 1B1u configurations, constructed from 31 898 Slater determinants. The lowest total energy obtained was −78.0284 hartree, significantly lower than any previous variational result. A key to determining the spatial extent of the vertical V state was provided in the important paper by McMurchie and Davidson, who showed how critical it is that the CI wavefunction be invariant to transformations within the π* (b3u) orbital space. However the actual value of 〈ψ‖Σi x2i‖ψ〉 obtained here, ∼27 a.u., is somewhat larger than that of McMurchie and Davidson. We conclude that the V state is intermediate in its spatial extent between valence and Rydberg character. However, it is also seen that variational energies are very insensitive to the value of 〈x2〉 predicted. That is, by raising the vertical excitation energy of the V state by only 0.1 eV, the intermediate spatial extent suggested by the variational principal becomes markedly more valencelike. The latter observation serves to explain the experimental spectra of Miron, Raz, and Jortner.