convert pair style bop to class2

\documentclass[12pt]{article}

\begin{document}

$$

  E = \frac{1}{2} \sum_{i=1}^{N} \sum_{j=i_1}^{i_N} \phi_{ij} \left( r_{ij} \right) - \sum_{i=1}^{N} \sum_{j=i_1}^{i_N} \beta_{\sigma,ij} \left( r_{ij} \right) \cdot \Theta_{\sigma,ij} - \sum_{i=1}^{N} \sum_{j=i_1}^{i_N} \beta_{\pi,ij} \left( r_{ij} \right) \cdot \Theta_{\pi,ij} + U_{prom}

$$

\end{document}

\documentstyle[12pt]{article}

\begin{document}

$$

E = A \exp \left(\frac{\sigma - r}{\rho} \right) - 

    \frac{C}{r^6} + \frac{D}{r^8} \qquad r < r_c

$$

\end{document}