Wigner-Seitz radius ($r_s$)

The radius $r_s$ of a sphere containing one electron at a specified density: $4\pi r_s^3\rho /3=1$.

Write $(3/(4\pi n){)}^{1/3}=r_s{a}_B^{\ast }$, where $a_B^{\ast }=4\pi \epsilon {\hslash }^2/(m^{\ast }{e}^2)$. Here $r_s$ is dimensionless, unlike a Wigner-Seitz radius measured as a length. Electron-gas energy formulas quoted in effective atomic units commonly use this convention.