Coulomb coupling ($\Gamma =\frac{e^2}{4\pi {ϵ}_{0}a{k}_{B}T}$)

The importance of electrostatic interaction energy compared with thermal energy. For mean separation $a$, a common measure is $\Gamma =e^2/(4\pi {ϵ}_{0}a{k}_{B}T)$; the weakly coupled regime has $\Gamma \ll 1$.