\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
W_{\rm pre-nb}(i) =
\sum_{j=1}^{\bf nch\_pre\_nb}
\left[
{
\left[\prod_{a} W_{\rm bond}\left(j,a\right)\right]
\left[\prod_{b} W_{\rm bend}^{\rm A}\left(j,b\right)\right]
\left[\prod_{c} W_{\rm bend}^{\rm B}\left(j,c\right)\right]
W_{\rm dihed}\left(j\right)
}
\right]
\end{displaymath}
\end{document}