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\begin{eqnarray*}
 E & = & 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - 
                       \left(\frac{\sigma}{r}\right)^6 \right]
                       \qquad r < r_{in} \\
 F & = & C_1 + C_2 (r - r_{in}) + C_3 (r - r_{in})^2 + C_4 (r - r_{in})^3 
                       \qquad r_{in} < r < r_c
\end{eqnarray*}                           

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