(5.4.6) Gori-Giorgi : I fully agree with everything said above, especially on using the constrained-search approach instead of introducing the Hohenberg-Kohn theorem. Obstacles I often see students facing when trying to learn DFT are: 1. To understand the theory behind DFT, you need to have a good understanding of many-electron wave functions and reduced density matrices, especially in real space, which they often lack.