(2.1.28) Chattaraj : Any theory that applies density to understand a many-particle system, without using the exact wave function, can be termed as DFT. According to Hohenberg-Kohn theorems, DFT is a theory that legitimizes the use of the density to calculate all possible properties. The Hohenberg-Kohn theorems are just existence theorems and do not provide any know-how for an explicit form of the energy as a functional of the density as well as functional forms of other properties.