\documentclass[12pt]{article}
\begin{document}
$$
E = K (r - R_c)^ 2 (r - R_c - B_1) (r - R_c - B_2) + U_0 +
4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} -
\left(\frac{\sigma}{r}\right)^6 \right] + \epsilon
\end{document}