Added equation for lj_cubic

\begin{eqnarray*}

 E &=& u_{LJ}(r) \qquad r \leq r_s \\

    &=&  u_{LJ}(r_s) + (r-r_s) u'_{LJ}(r) - \frac{1}{6} A_3 (r-r_s)^3 \qquad r_s < r \leq r_c \\

    &=&  u_{LJ}(r_s) + (r-r_s) u'_{LJ}(r_s) - \frac{1}{6} A_3 (r-r_s)^3 \qquad r_s < r \leq r_c \\

    &=& 0 \qquad r > r_c 

\end{eqnarray*}