(2.2.8) Staroverov : The classic Kohn-Sham scheme almost solves the problem of the kinetic-energy functional but its onedeterminantal form creates formidable challenges for approximating the exchange-correlation part. These include the difficulty of devising exchange-correlation functionals for strongly correlated systems (see contribution (3.4.1)), limitations imposed by the assumption of non-interacting v-representability by a single Slater determinant, and the intricate behaviour of exact Kohn-Sham potentials (e.g., shifts within nodal surfaces of the highestoccupied Kohn-Sham orbital 80 ), which DFAs somehow have to get just right. Although the existing Kohn-Sham DFAs are amazingly more accurate than the Hartree-Fock method in general, it is sobering that they still inherit most qualitative failures (see 3.4) of the mean-field approximation. Ensemble methods (see Section 3.7) seem unavoidable from this perspective.