Comparison of density functional and Hartree-Fock average local ionization energies on molecular surfaces

We investigated whether an average local ionization energy defined within the framework of Hartree–Fock theory is physically meaningful in Kohn–Sham density functional theory, in view of the uncertainty in relating the latter orbital energies to electronic ionization energies. In a preliminary study of N2O using several different exchange/correlation functional combinations, it was found that the orbital energies in each case correlate roughly with experimental binding energies. Average local ionization energies are computed on the molecular surfaces of 11 monosubstituted benzene derivatives using both Hartree–Fock and density functional procedures. Both sets of results correctly predict the ortho/para- or meta-directing tendencies of the substituents, as well as their ring-activating or -deactivating effects. Excellent correlations were obtained with the Hammett substituent constants. This initial study suggests that Kohn–Sham average local ionization energies should be physically meaningful, on a relative basis.