Walter Kohn showed in 1964-65 that the energy of a quantum-mechanical system is uniquely determined by its electron density. This quantity is more easily handled than the complicated wave function in the Schrödinger equation. Kohn also provided a method which made it possible to set up equations whose solutions give the system’s electron density and energy. This method, called density functional theory, has become widely used in chemistry since, because of its simplicity, it can be applied to fairly large molecules.