\begin{equation}
\begin{split}
\mf{f}^p_{ij}(t) = &\left\{
\begin{array}{rl}
\epsilon(1-\frac{r_{ij}(t)}{2r_0})^{3/2}\hat{r}_{ij}(t) &\text{if } r_{ij} < 0.5 \\
0 & \text{otherwise,}
\end{array} \right. \\
\mf{f}^p_i(t) =&\sum_j\mf{f}^p_{ij} \text{, where}
\end{split}
\label{eqn:phys}
\end{equation}