Ab initio calculations have been carried out for the T(3B1u) and V(1B1u) states of ethylene in a planar nuclear configuration. A "double-zeta" gaussian basis, augmented by diffuse functions, was used. A wide variety of configuration interaction wavefunctions (including as many as 1605 configurations), optimized by the iterative natural orbital method, were computed. The calculations predict the1B1ustate to lie 3.8 - 4.1 eV above the3B1ustate. The spatial extent of the triplet state is that of a normal valence state with (x2) ≈ 12 bohr2. The singlet state, on the other hand, is found to be quite diffuse or Rydberg-like, (x2) ≈ 35 bohr2; electron correlation was found to decrease (x2) by 30% from the Hartree - Fock value (52 bohr2). This result is not consistent with simple notions concerning π-electron theory. © 1972.
Bender, C. F., Dunning, T. H., Schaefer, H. F., Goddard, W. A., & Hunt, W. J. (1972). Multiconfiguration wavefuntions for the lowest (ππ*) excited states of ethylene. Chemical Physics Letters, 15(2), 171–178. https://doi.org/10.1016/0009-2614(72)80143-X