(2.1.4) Scheffler : Since the development of the quantum mechanics of atoms and polyatomic systems, it was clear that inspection of the ground-state electron density ρ(r) provides the information on the total number of electrons, N, the positions of the atoms, {R I }, and from ρ(R I ) the nuclear charges . Thus, ρ(r) determines N, {R I }, {Z I } -that is, the many-electron Hamiltonian, and therefore, it determines everything. This is the algorithm that defines how to go from the ground-state density to the energy.