\documentclass[12pt]{article}
\begin{document}
$$
E=\sum_{ij}\phi(r_{ij})+\sum_{i}U(\rho_{i}),
\rho_{i}=\sum_{j}\rho(r_{ij})+\sum_{jk}f(r_{ij})f(r_{ik})g[\cos(\theta_{jik})]
\end{document}