An energy-independent effective hamiltonian for ethylene

Abstract

The effective one- and two-electron π hamiltonians for ethylene are constructed using the quasi-degenerate Rayleigh-Schrödinger perturbation method of Brandow [1, 2]. From these calculations the usual semi-empirical parameters α, β, γ₀₀ and γ₀₁ are obtained as well as the excitation energies and ionization potential.

Acceptable numerical results are obtained only when diffuse orbitals are added to the basis and the π orbitals are optimized using multi-configuration SCF (MC-SCF) methods. It is demonstrated that correlation has only a small effect on the excitation energies and that the major contribution of electron correlation is to scale the atomic parameters and thus effectively reduce the diffuseness of the π orbitals. These scaled parameters are still consistent with empirical recipes for evaluation from atomic data.