A uniform electron gas at zero temperature is characterised by a single dimensionless parameter, the so-called Wigner–Seitz radius r s = a / a b, where a is the average inter-particle spacing and a b is the Bohr radius. The kinetic energy of an electron gas scales as 1/ r s, this can be seen for instance by considering a simple Fermi gas. The potential energy, on the other hand, is proportional to 1/ r s. When r s becomes larger at low density, the latter becomes dominant and forces the electrons as far apart as possible. As a consequence, they condense into a close-packed lattice. The resulting electron crystal is called the Wigner crystal.