\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} W_{\rm bond}(n,a) = \sum_{i=1}^{\bf nch\_vib} { \left[ {{p_{\rm trial}^{\rm ideal}\left(l_{\rm g_a,f}(i)\right)}\over{ p_{\rm trial}^{\rm arb}\left(l_{\rm g_a,f}(i)\right)}} \exp\left[-\beta u_{\rm bond}\left(l_{\rm g_a,f}(i)\right)\right] \right] } \end{displaymath} \end{document}