(2.2.12) Vignale : A question that keeps resurfacing is: Why are the Kohn-Sham orbitals better than the Hartree-Fock orbitals? From the point of view of the variational principle, the Hartree-Fock orbitals should be the best, since they build a Slater determinant which has the lowest energy (defined as expectation value of the Hamiltonian) among all Slater determinants. The Kohn-Sham wave function -also a single Slater determinantcannot beat that. Nevertheless, we know that the DFT energy is better than the Hartree-Fock energy and also that the Kohn-Sham orbitals, as discussed in contribution (2.2.4), far from being meaningless, are in many ways "better" than the Hartree-Fock orbitals.