1. Wigner–Seitz radius

In a 3-D system with particles in a volume , the Wigner–Seitz radius is defined by^[1]

Solving for we obtain

where is the particle density of the valence electrons.

For a non-interacting system, the average separation between two particles will be . The radius can also be calculated as

where is molar mass, is mass density, and is the Avogadro number.

This parameter is normally reported in atomic units, i.e., in units of the Bohr radius.

Values of for single valence metals^[2] are listed below:

Element
Li	3.25
Na	3.93
K	4.86
Rb	5.20
Cs	5.62