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\begin{eqnarray*}
 E & = & \frac{Cq_iq_j}{\epsilon r} \left( \frac{\epsilon}{\epsilon_D(r)}-1\right)                       \qquad r < r_c \\
 \epsilon_D(r) & = & \frac{5.2+\epsilon}{2} +  \frac{\epsilon-5.2}{2}\tanh\left(\frac{r-r_{me}}{\sigma_e}\right) 
\end{eqnarray*}
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