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\begin{equation*}
J_\text{ep} = -\sum_{i=1}^2 \sum_{j=1}^2 \frac{a_\text{ep} |\mathbf{r}_i - \mathbf{I}_j|}{1 + b_\text{ep} |\mathbf{r}_i - \mathbf{I}_j|},
\end{equation*}
\end{document}