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+\documentclass[12pt]{article}
+
+\begin{document}
+
+\begin{eqnarray*}
+  U & = & \sum_i^N \sum_{j > i}^N U_{ij}^{(2)} (r_{ij}) + 
+          \sum_i^N \sum_{j \neq i}^N \sum_{k > j, k \neq i}^N 
+          U_{ijk}^{(3)} (r_{ij}, r_{ik}, \theta_{ijk}) 
+          \\
+  U_{ij}^{(2)} (r) & = &  \frac{H_{ij}}{r^{\eta_{ij}}}
+  	                        + \frac{Z_i Z_j}{r}\exp(-r/\lambda_{1,ij})
+  	                        - \frac{D_{ij}}{r^4}\exp(-r/\lambda_{4,ij})
+	                        - \frac{W_{ij}}{r^6}, r < r_{c,{ij}}
+                   \\
+  U_{ijk}^{(3)}(r_{ij},r_{ik},\theta_{ijk}) & = & B_{ijk} 
+                    \frac{\left[ \cos \theta_{ijk} - \cos \theta_{0ijk} \right]^2}
+                    {1+C_{ijk}\left[ \cos \theta_{ijk} - \cos \theta_{0ijk} \right]^2} \times \\
+&  &                     \exp \left( \frac{\gamma_{ij}}{r_{ij} - r_{0,ij}} \right)
+                    \exp \left( \frac{\gamma_{ik}}{r_{ik} - r_{0,ik}} \right), r_{ij} < r_{0,ij}, r_{ik} < r_{0,ik}
+\end{eqnarray*}                           
+
+\end{document}