Hard sphere model

The hard sphere (HS) interaction potential is given by

Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle 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.