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