(2.1.22) Krylov : I first learned about the key ideas behind DFT before its modern incarnation was developed. Back in the eighties, chemists were using the Xα method, which was regarded by ab initio theorists as semiempirical and, therefore, inferior to then-gold-standard -the full Hartree-Fock method. We were struggling to understand why an inferior method would give more accurate results. I think the real insight was to understand that the Wilson conjecture -the observation that the one-electron density contains all the information needed to reconstruct the many-body Hamiltonian (and, therefore, to find the exact solution of the Schrödinger equation) -provides a physical justification for the existence of a mapping between the density and the exact energy of the system. The Hohenberg-Kohn theorems inform us that this mapping is unique.