\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}