(3.5.3) Baerends : One consequence of the orbital levels for most DFAs being much too high (see contribution (3.2.13)) is a much too high-lying LUMO level. Adding an extra electron to the system then causes the LUMO level (containing the added electron) to be so close to zero that the LUMO becomes very diffuse, or even above zero, with an infinitely extended LUMO. This is a case where the Hartree-Fock method yields a much improved density. A similar effect can be achieved by using a more accurate model Kohn-Sham potential. On the other hand, while the Hartree-Fock density is typically very good for atoms, the Hartree-Fock model often yields poor bonding densities for molecules, being too diffuse around nuclear centres. For instance, for H 2 it has been demonstrated that, due to this diffuse character, the errors in the one-electron energy terms (not sufficiently negative electron-nuclear energy and too low kinetic energy) are comparable to the error in the electron-electron energy. Upon stretching H 2 , the errors in the one-electron energy terms soon exceed the two-electron energy errors. For N 2 , the one-electron errors due to the too diffuse Hartree-Fock density are at the equilibrium distance already larger than the total bond energy of about 10 eV! These errors can be understood from the limited flexibility of the Hartree-Fock wave function -that is, they arise from the lack of electron correlation. This gives a strong incentive to develop accurate model Kohn-Sham potentials that do better for the density and for the orbital energies and is also very important for the MO-theoretical explanations in chemistry.