(2.2.15) Yang : I would like to address the physical meaning of Kohn-Sham orbitals in calculations with DFAs. Most DFAs to the exchange-correlation energy E xc usually produce reasonable total energies for small and medium-size molecules; however, they have major deficiencies in the orbital energies. As has been known for a long time, for finite systems, the eigenvalue of the HOMO for the exact Kohn-Sham potential is equal to the negative of the first ionization potential (IP), as follows from the asymptotic decay behaviour of the exact electron density and the requirement that the Kohn-Sham effective potential be zero at infinity. However, in a Kohn-Sham calculation, the local Kohn-Sham potential can have any additive constant and give the same total energy and density but different orbital energies. Thus, the argument based on the long-range behaviour of density and potential hinges on a particular choice of the additive constant of the potential.