convert remaining lj pair styles

\documentstyle[12pt]{article}

\begin{document}

$$

 E = 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - 

                       \left(\frac{\sigma}{r}\right)^6 \right]

                       \qquad r < r_c

$$

\end{document}

\documentstyle[12pt]{article}

\begin{document}

$$

 E = 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{9} - 

                       \left(\frac{\sigma}{r}\right)^6 \right]

                       \qquad r < r_c

$$

\end{document}