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\begin{eqnarray*}
  E & = & \sum_i \sum_{j > i} \phi_2 (r_{ij}) + 
          \sum_i \sum_{j \neq i} \sum_{k > j} 
          \phi_3 (r_{ij}, r_{ik}, \theta_{ijk}) \\
  \phi_2(r) & = & A \epsilon \left[ B (\frac{\sigma}{r})^p - 
                    (\frac{\sigma}{r})^q \right] 
                    \exp \left( \frac{\sigma}{r - a \sigma} \right) \\
  \phi_3(r,s,\theta) & = & \lambda \epsilon \left[ \cos \theta - 
                    \cos \theta_0 \right]^2
                    \exp \left( \frac{\gamma \sigma}{r - a \sigma} \right)
                    \exp \left( \frac{\gamma \sigma}{s - a \sigma} \right)
\end{eqnarray*}                           

\end{document}