Harmonic repulsion potential

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

\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 $\epsilon =20$ the potential looks like