Non-linear density functional theory: A direct method to calculate many-electron charge densities

We suggest to include the density of electron charge explicitly in the electron potential of density functional theory, rather than implicitly via exchange-correlation functionals. The advantages of the approach are conceptual and numerical. Conceptually, it allows to formulate a physical principle for the development of quantum mechanical systems: it is the principle of energetic equilibrium, because the energy principle, in this case, applies not only globally, but also on a local level. The method is an order-$N$ method, scaling linearly with the number of atoms. It is used to calculate the electronic groundstate of a metallic surface, where we find good agreement with experimental values.