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\begin{eqnarray*}
  F_i^u & = & \sum_t^{N_t}\alpha_t \cdot \exp\left[-\frac{\left(d_{i,t}^u\right)^2}{2l^2}\right] \\
  d_{i,t}^u & = & \left|\left| V_i^u(\eta) - V_t^u(\eta) \right|\right| \\
  V_i^u(\eta) & = & \sum_{j \neq i}\frac{r^u_{ij}}{r_{ij}} \cdot e^{-\left(\frac{r_{ij}}{\eta} \right)^2} \cdot f_d\left(r_{ij}\right) \\
  f_d\left(r_{ij}\right) & = & \frac{1}{2} \left[\cos\left(\frac{\pi r_{ij}}{R_c}\right) + 1 \right]
\end{eqnarray*}

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