The physics of the zero-temperature phase behavior of jellium is driven by competition between the kinetic energy of the electrons and the electron-electron interaction energy. The kinetic-energy operator in the Hamiltonian scales as 1 / r s 2 , where r s is the Wigner–Seitz radius, whereas the interaction energy operator scales as 1 / r s . Hence the kinetic energy dominates at high density (small r s ), while the interaction energy dominates at low density (large r s ).