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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