\documentclass[12pt]{article}

\begin{document}

\begin{eqnarray*}
  E & = & E_a + E_{bb} + E_{ba} \\
  E_a & = & K_2 (\theta - \theta_0)^2 + K_3 (\theta - \theta_0)^3 + K_4 (\theta - \theta_0)^4 \\
  E_{bb} & = & M (r_{ij} - r_1) (r_{jk} - r_2) \\
  E_{ba} & = & N_1 (r_{ij} - r_1) (\theta - \theta_0) + N_2 (r_{jk} - r_2) (\theta - \theta_0)
\end{eqnarray*}

\end{document}