\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
W_{\rm bend}^{B}(n,b) =
\sum_{i=1}^{\bf nch\_bend\_b}
\left[
{{p_{\rm trial}^{\rm ideal}\left(\phi_{g_b,f,g_a}(i)\right)}\over{ p_{\rm trial}^{\rm arb}\left(\phi_{g_b,f,g_a}(i)\right)}}
\exp\left[-\beta u_{\rm bend}\left(\theta_{g_b,f,g_a}(i)\right)\right]
\exp\left[-\beta u_{\rm imp}\left(\phi_{p,f,g_a,g_b}(i)\right)\right]
\right]
\end{displaymath}
\end{document}