Harmonic repulsion potential

The Harmonic repulsion potential, designed for studies of polymer micelles, dendrimers, and colloidal particles, is given by (^[1] Eq. 2):

Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle \Phi _{12}\left(r\right)=\left\{{\begin{array}{lll}{\frac {\epsilon }{2}}(1-r/\sigma )^{2}&;&r<\sigma \\0&;&r\geq \sigma \end{array}}\right.}

where $\Phi _{12}\left(r\right)$ is the intermolecular pair potential, $r:=|\mathbf {r} _{1}-\mathbf {r} _{2}|$ is the distance between site 1 and site 2. $\sigma$ represents a length scale and $\epsilon$ an energy. For example, for Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle \epsilon =20} the potential looks like