A theoretical study of the diffuseness of the V(1B1u) state of planar ethylene

The ground (N) state and the ¹B_1u(V) excited state of planar ethylene have been studied at the CPF and MR -SDCI levels of theory, using an extended CGTO basis set of the ANO type. The investigation especially addresses the near-degeneracy problem in the ground state and the coupling between the diffuse character of the π* orbital and the amount of correlation included in the wave function of the V state. The MR -SDCI results yield a vertical excitation energy in the range 7.8–8.0 eV, whereas the CPF result is 7.9 eV. The best MR -SDCI result for 〈1π*‖z²‖1π*〉 is 7.8, whereas CPF calculations based on MR -SDCI INOS give the value 6.7. It is clear from the results that these numbers have not converged and that more extended calculations than was possible in the present work would yield an even more compact wave function.