Effective one-electron potential in the Kohn-Sham molecular orbital theory

Density functional theory has received great interest mostly because of the accurate bonding energies and related properties (geometries, force constants) it provides. However, he Kohn-Sham molecular orbital method, that is almost exclusively used, is more than a convenient tool to generate the required electron density. The effective one-electron potential in the Kohn-Sham equations is intimately related to the physics of electron correlation. We demonstrate that it is useful to break down the exchange-correlation part of the potential into a part that is directly related to the total energy (the hole potential or screening potential) and a socalled response part that is related to ''response'' of the exchange-correlation hole to density change. The latter part is poorly represented by the generalized gradient approximation, explaining why this approximation yields accurate total energies but fails for simple orbital related quantities such as the HOMO orbital energy. A simple modelling of the response potential is proposed. We stress the usefulness of Kohn-Sham orbitals in chemistry, both quantitatively (by providing in principle the exact electron density, to be used in density functionals for the energy) and qualitatively (for use in qualitative MO theory).