\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} W_{\rm nb}(n) = \sum_{i=1}^{\bf nch\_nb} \left[ \exp\left(-\beta \sum_{a=1}^{n_{\rm g}(n)} u_{\rm nb}^{\rm part}\left({\bf r}_a(i)\right) \right) W_{\rm dihed}(i) \right] \end{displaymath} \end{document}