The Equivalent Bond Orbital Model Revisited: I. Orbitals, Orbital Energies and PE Spectra of Saturated Hydrocarbons

The EBO model, originally proposed by Lennard-Jones and Hall, many years ago, is reexamined using localized orbitals (LMO) stemming from SCF ab initio calculations. It is found that only a· limited set of matrix elements is needed to build a Fock matrix F, (in localized basis) the diagonalization of which yields orbital energies of sufficient precision for use in the assignment of photoelectron spectra within Koopmans e1pproximation. The model so obtained can be easily parametrized with respect to a given calibration set, if the relative size of the off-diagonal elements suggested by the ab initio calculation is conserved. It is argued that such EBO models, which are essentially of Hiickel-type, Cl'!'e necessary for the qualitative or semi-quantitative rationalization of experimental and theoretical data (from more sophisticated calculations). Some of the more important limitations of LMO-based EBO models ctre discussed.