\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
W_{\rm nb}\left(n\right) = 
\sum_{i=1}^{\bf nch\_nb}
\left[
{
{
\exp\left(-\beta {\sum_{a=1}^{n_{\rm g}\left(n\right)}} u_{\rm nb}^{\rm part}\left({\bf r}_a\left(i\right)\right)\right)
{
W_{\rm pre-nb}(i)
}
}
}
\right]
\end{displaymath}
\end{document}