This Demonstration considers three well-known models for intermolecular potentials: square-well, Suthlerand, and Lennard-Jones. Potentials for molecule A (red), and molecule B (blue), and for an A-B mixture (dashed purple) are shown.

The variable is the square-well potential parameter. The binary interaction parameter attempts to represent non-idealities in the mixture, with a value of 0 for the ideal case. The attractive-strength parameter is the ratio of the attractive strength of molecule B to that of molecule A. The molecular-size parameter is the ratio of the diameters of molecules B and A.

Here is the molecular diameter, is the distance between the molecule centers, is the intermolecular potential, and is the depth of the potential well.

The attractive strength parameter of the mixture is given by

.

A larger negative number for the binary interaction parameter represents stronger interactions between A and B relative to those between A and A or B and B. Analogously, a smaller positive number represents weaker interactions.

The molecular size parameter of the mixture is taken as