(3.2.14) Görling : The origin of the errors that most DFAs exhibit for the Kohn-Sham orbital energies is the presence of unphysical self-interactions. A solution to this problem has been around for a long time -namely, an exact treatment of the Kohn-Sham exchange potential, which requires the OEP method. If the exact local Kohn-Sham exchange potential -that is, the OEP exchange potential -is used, then the HOMO eigenvalue immediately is close to the IP as it should, whereas, in conventional GGA calculations, it is typically several eV to high. Moreover, the Kohn-Sham eigenvalue spectrum changes qualitatively: An exact-exchange (EXX-OEP) calculation gives a Rydberg series as it should, while a GGA calculation does not. Thus, if the selfinteraction contained in the Hartree potential is properly cancelled by the exact Kohn-Sham exchange potential, then a qualitatively correct and quantitatively much more accurate spectrum of Kohn-Sham orbital energies is obtained. The OEP method has a bad reputation because of numerical problems. However, these problems have been solved and computationally efficient, numerically stable OEP methods are now available. An exact treatment of exchange requires correlation functionals that go along with it. Such correlation functionals exist -for example, RPA-based functionals 102-104 -but are so far not very popular. The poor orbital energies are thus the price to pay for approximating exchange and correlation together in most DFAs, in order to exploit error cancellations.