ABSTRACT: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 N O using several different 2 exchangercorrelation 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 orthorpara-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.

Hartree-Fock and Kohn-Sham orbital energies, the latter computed with several dierent exchange/ correlation functionals, are compared and analyzed for 12 molecules. The Kohn-Sham energies dier signi®-cantly from experimental ionization energies, but by amounts that are, for a given molecule and exchange/ correlation functional, roughly the same for all of the valence orbitals. With the exchange/correlation functionals used, the energy of the highest occupied KohnSham orbital does not approximate the corresponding ionization potential any better than do the other orbital energies.

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