Variational Calculation of Ethylene

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Abstract
A variational treatment is applied to the lower electronic states of planar ethylene; specifically the 1Ag(N), 3B1u(T), and 1B1u(V) states are investigated. The problem is treated nonempirically by using the method of molecular orbitals within the framework of the π‐electron approximation. Several new features are introduced. First, we use separate ζ values (orbital exponents) for the bonding and antibonding LCAO MO's. This brings about significant improvements in lowering the energy of the V state. As a result, the separation of the V and T states, which was too large in the conventional LCAO MO calculations, is now in much better agreement with the experiment. Extensive variations are made in the π‐electron orbitals; three different σ‐core potential fields are also examined. Zener's value of the effective charge for the σ‐core electrons is shown to be an adequate choice for characterizing this core field. The effect of the four hydrogen atoms is also investigated.

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