  We give a novel and simple proof of the DFT expression for the interatomic
force field that drives the motion of atoms in classical Molecular Dynamics,
based on the observation that the ground state electronic energy, seen as a
functional of the external potential, is the Legendre transform of the
Hohenberg-Kohn functional, which in turn is a functional of the electronic
density. We show in this way that the so-called Hellmann-Feynman analytical
formula, currently used in numerical simulations, actually provides the exact
expression of the interatomic force.
