\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}