Here the contributions are the kinetic energy of the electrons 𝑇[𝜌 0 ], the potential energy from electron-electron repulsion 𝑉 ee [𝜌 0 ] and the potential energy from electron-nuclear attraction 𝑉 ne [𝜌 0 ]. A functional is a function where the argument is again a function (while the argument in a function is a scalar quantity). Since the electron density is a function of the cartesian coordinates (i.e., it varies at different points in 3D space), this makes the energy a functional of 𝜌. This theorem allows to compute the energy and all properties of a system just by knowing the electron density.