Square shoulder + square well model

The Square shoulder + square well model consists, as its name suggests, of a combination of the square shoulder model and the square well model. The model is given by

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

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Phi_{12}(r)} is the intermolecular pair potential, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \epsilon_1} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \epsilon_2} are the well depths, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 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 Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \lambda_1,\lambda_2>1}

Critical points and liquid-liquid transition

For certain parameter sets the square shoulder + square well model has a liquid-liquid transition along with a corresponding liquid-liquid critical point .

References

Related reading

• W. Rzysko, O. Pizio, A. Patrykiejew, and S. Sokolowski "Phase diagram of a square-shoulder, square-well fluid revisited", Journal of Chemical Physics 129 124502 (2008)
• Shiqi Zhou, A. Lajovic, and A. Jamnik "Local structures of fluid with discrete spherical potential: Theory and grand canonical ensemble Monte Carlo simulation", Journal of Chemical Physics 129 124503 (2008)