A thermodynamic model used in phase equilibria calculations that correlates the activity coefficients of a component in a liquid mixture with its mole fraction. It is based on the hypothesis that the concentration around a molecule is different from the bulk concentration caused by the difference in interaction energy of the central molecule with the molecules of its own kind and that of the molecules of the other kind. The energy difference also introduces a non-randomness at the local molecular level. The model belongs to the local-composition type which includes the Wilson, UNIQUAC, and UNIFAC models.