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.