Hard sphere model

The hard sphere (HS) interaction potential is given by

$V\left(r\right)=\left\{{\begin{array}{lll}\infty &;&r<\sigma \\0&;&r\geq \sigma \end{array}}\right.$

where $V\left(r\right)$ is the potential energy between two spheres at a distance $r$, and $\sigma$ is the sphere diameter.