Double square well model

The double square well model 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 $\lambda _{1},\lambda _{2}>1$

References

1. M. Reza Sadr-Lahijany, Antonio Scala, Sergey V. Buldyrev, and H. Eugene Stanley "Liquid-State Anomalies and the Stell-Hemmer Core-Softened Potential", Physical Review Letters 81 pp. 4895-4898 (1998)
2. J. R. Solana "Thermodynamic properties of double square-well fluids: Computer simulations and theory", Journal of Chemical Physics 129 244502 (2008)