(2.1.10) Yang : The Hohenberg-Kohn work established the principles for describing a many-electron system from the reduced variable of its electron density and the Kohn-Sham work provided the formulation to use a noninteracting reference system to represent the electron density of a many-electron system. These works are the solid foundation of DFT. However, they do not lead to any systematic pathway to the approximation of the density functional; see contribution (2.1.8). The specific approximations for the density functionals are the key to all applications.