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