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+\documentclass[12pt]{article}
+
+\begin{document}
+$$
+  E_i = \frac{1}{2} \sum_{j \neq i} 
+  \frac{q_i q_j {\rm erfc}(\alpha r_{ij})}{r_{ij}} + 
+  \frac{1}{2} \sum_{j \neq i} 
+  \frac{q_i q_j {\rm erf}(\alpha r_{ij})}{r_{ij}} \qquad r < r_c
+$$
+
+\end{document}