\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
P_{\rm select}^{\rm bond}\left(l_{\rm g_a,f}(i)\right) = 
{{
\left[
{{p_{\rm trial}^{\rm ideal}\left(l_{\rm g_a,f}(i)\right)}\over{ p_{\rm trial}^{\rm arb}\left(l_{\rm g_a,f}(i)\right)}}
\exp\left[-\beta u_{\rm bond}\left(l_{\rm g_a,f}(i)\right)\right]
\right]
}
\over
{
W_{\rm bond}(n,a)
}}
\end{displaymath}
\end{document}