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

Ab initio calculations have been carried out for the T(^(3)B_(1u)) and V(^(1)B_(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 ^(1)B_(1u) state to lie 3.8 – 4.1 eV above the 3B1u state. The spatial extent of the triplet state is that of a normal valence state with (x2) ≈ 12 bohr^2. The singlet state, on the other hand, is found to be quite diffuse or Rydberg-like, (x^2) ≈ 35 bohr^2; electron correlation was found to decrease (x^2) by 30% from the Hartree - Fock value (52 bohr^2). This result is not consistent with simple notions concerning π-electron theory.