The predecessor to density functional theory was the Thomas–Fermi model, developed independently by both Llewellyn Thomas and Enrico Fermi in 1927. They used a statistical model to approximate the distribution of electrons in an atom. The mathematical basis postulated that electrons are distributed uniformly in phase space with two electrons in every h 3 of volume. For each element of coordinate space volume d 3 r we can fill out a sphere of momentum space up to the Fermi momentum p F