The foundation of DFT is formed by the two theorems formulated by Hohenberg and Kohn in 1964. Their core idea was to use the electron density 𝜌 as the central quantity in formulating their theory of electronic structure in contrast to using the wave function (section 2.2). Previous approaches by Thomas and Fermi have already been formulated in 1927 but due to the use of classical terms for electron-electron interaction and the kinetic energy of the electrons, covalent bonding could not be described accurately. The first Hohenberg-Kohn theorem states that the total ground energy 𝐸 0 of a system is a unique functional of the electron density: