Multiconfiguration wavefuntions for the lowest (ππ*) excited states of ethylene

Ab initio calculations have been carried out for the T(³B_1u) and V(¹B_1u) 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 the ¹B_1u state to lie 3.8 - 4.1 eV above the ³B_1u state. 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.