\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} W_{\rm dihed}(j) = \sum_{k=1}^{\bf nch\_tor} \left[ { {{p_{\rm trial}^{\rm ideal}\left(\phi_{g_1,f,p,q_b}(k)\right)}\over{ p_{\rm trial}^{\rm arb}\left(\phi_{g_1,f,p,q_b}(k)\right)}} w_{\rm bias}^{\rm fe}(k) \exp\left[-\beta \sum_{a=1}^{n_{\rm g}(n)} u_{\rm con}\left(\phi_{g_a,f,p,q_b}(k)\right)\right] } \right] \end{displaymath} \end{document}