Interpretation of open-shell SCF calculations on the T and V states of ethylene

Self-consistent field calculations are reported on (planar and perpendicular geometric conformers of) the lowest energy triplet and singlet (π, π^*) orbital configurations of ethylene, which are usually identified with the spectroscopic T and V states, respectively. For the planar conformation the calculation predicts a V state with the characteristics of a Rydberg state, but a T state of expected valence-shell character. The π^* orbital is much too large and the internuclear distance too small for the calculated V state, but are as expected for the T state. It is concluded that the calculated result for the supposed V state in the planar conformation is spurious due to the inadequacy of the Hartree-Fock single-configuration theory. The supposed V state here calculated may be identified with a Rydberg state which has been observed spectroscopically at ̃1.4 eV higher energy than the V state in the optical spectrum. For the perpendicular conformation, the calculations yield resonable results for both the T and V states. Calculations also have been carried out on a singlet (σ, π^*) state for planar ethylene; here the size of the π^* orbital is reasonable.