\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
u_{\rm con}\left(\phi_{g_a,f,p,q}\right) =
\sum_{c \neq f} u_{\rm bond}(l_{g_a,c})
+ \sum_{c \neq f} u_{\rm bend}(\theta_{g_a,c,d})
+ \sum u_{\rm bend}(\theta_{c,g_a,d})
+ \sum u_{\rm tor}(\phi_{g_a,c,d,e})
\end{displaymath}
\begin{displaymath}
+ \sum u_{\rm tor}(\phi_{c,g_a,d,e})
+ \sum u_{\rm imp}(\phi_{g_a,f,c,d})
+ \sum_{c \neq f} u_{\rm imp}(\phi_{c,g_a,d,e})
\end{displaymath}
\end{document}