\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}