Soft-core square well model

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The soft-core square well model is given by

$\Phi_{12}\left( r \right) = \left\{ \begin{array}{lcc} 4 \epsilon \left[ \left(\frac{\sigma}{r} \right)^{n}- \left( \frac{\sigma}{r}\right)^{n/2} \right] & ; & r \leq r_m \\ - \epsilon & ; &r_m < r \leq \lambda \sigma \\ 0 & ; & r >\lambda \sigma \end{array} \right.$

where $Φ 12 (r)$ is the intermolecular pair potential, $r m = 2 2 / n σ$ , $ε$ is the well depth, $r$ is the distance between site 1 and site 2, i.e. $r := |\mathbf{r}_1 - \mathbf{r}_2|$ , σ is the hard diameter and λ > 1.