\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}

$$
   E=\sum_{i<j}\phi(r_{ij})+\sum_{i}U(n_{i}),
$$

$$
   n_{i}=\sum_{j}\rho(r_{ij})+\sum_{\substack{j<k,\\j,k\neq i}}f(r_{ij})f(r_{ik})g[\cos(\theta_{jik})]
$$

\end{document}
