\documentclass[12pt]{article}

\begin{document}
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\begin{eqnarray*}
S_{ab} & = & - \left[ m (v_a - vave_a) (v_b-vave_b) + 
   \frac{1}{2} \sum_{n = 1}^{N_p} (r_{1_a} F_{1_b} + r_{2_a} F_{2_b}) 
    \right]
\end{eqnarray*}

\end{document}

