(3.1.5) Loos : The uniform electron gas, a hypothetical infinite substance where an infinite number of electrons "bathe" in a (uniform) positively charged jelly of infinite volume, is one of the success stories of DFT and, in particular, the parameterization of its correlation energy as a function of the density has been enormously useful for the construction of DFAs. From a more general point of view, model systems (especially the ones with uniform electron densities) provide new ways for improving and testing DFAs. In this regard, finite uniform electron gases (where electrons are confined to the surface of a sphere) can be seen as an extension of the conventional "infinite" version thanks to additional degrees of freedom coming from the tunable "finiteness" of the electron gas. (3.1.6) Reining : I would like to elaborate on the contribution of Loos in (3.1.5): using results from the uniform electron gas has been invaluable for the success of DFT. Here, we should stress how much DFT has profited from other people's work and methods -in particular, from the quantum Monte Carlo calcu-lations of This is important: trying to use the strong points of other methods -and trying to use the knowledge of model systems for the real materials we are interested in. This strategy could be extended much further. (3.1.7) Savin : An important decision in constructing DFAs is the choice of parts to be approximated by a closed form. Hohenberg and Kohn already considered it necessary to treat exactly the Hartree term, thus treating the electrostatic contribution to the energy correctly. Kohn and Sham decided to leave the DFA for exchange and correlation (Section II.A of their paper), or for correlation only (Section II.B). One can discuss having a part of exchange treated by orbitals and a part by DFAs, as done in hybrid DFT. One can also decide to treat only a part of correlation by a DFA -see, for example, ref. 243.