To change the system from one that interacts electromagnetically to one that interacts gravitationally, certain relationships should be defined. Large, comparatively unmoving bodies may act as the “nuclei”, such as the sun (this is a simplification since we are only considering our solar system). The planets, moons, and other celestial bodies could be the individual particles, with their gravitational interactions replacing the Hartree potential. The sun’s gravity would act as the external potential, and the initial kinetic energy is related to the bodies’ orbital velocities and mass. The exchange-correlation potential is similarly dubious, so it must be known prior to simulation.