(3.1.8) Staroverov : The analytic derivations of density functionals for model systems that gave us the Dirac exchange and Thomas-Fermi theory, as well as derivations of DFAs from model exchange-correlation holes have been seminal. The success of this analytic approach seems difficult to sustain in DFT, but that is almost certainly because not everything has been tried. Attempts to connect DFT with wave-function methods explicitly can also result in effective practical methods, especially for calculations of accurate Kohn-Sham potentials. (3.1.9) Johnson : Explicit modelling of the separate exchange, dynamical, and nondynamical correlation holes has been a very successful strategy in functional development. The (exact) exchange-correlation energy can be written in terms of the exchange-correlation hole as: