Double square well model

The double square well model ^[1] can be seen as a variant (or vice versa) of the square shoulder + square well model. In other words

\Phi _{12}\left(r\right)=\left\{{\begin{array}{ccc}\infty &;&r<\sigma \\-\epsilon _{1}&;&\sigma \leq r<\lambda _{1}\sigma \\-\epsilon _{2}&;&\lambda _{1}\sigma \leq r<\lambda _{2}\sigma \\0&;&r\geq \lambda _{2}\sigma \end{array}}\right.

where $\Phi _{12}(r)$ is the intermolecular pair potential, $\epsilon _{1}$ and $\epsilon _{2}$ are the well depths ( $\epsilon _{1},\epsilon _{2}>0$ ), $r$ is the distance between site 1 and site 2 where $r:=|\mathbf {r} _{1}-\mathbf {r} _{2}|$ , σ is the hard core diameter and $\lambda _{1},\lambda _{2}>1$