UNIFAC

thumb|250px|Saddle azeotrope calculated with UNIFAC at 1 atm. Red lines are vapor compositions and blue lines are liquid compositions. Image is rotating to more clearly show the saddle-like shape of the vapor–liquid equilibria

In statistical thermodynamics, the UNIFAC method (UNIQUAC Functional-group Activity Coefficients) is a semi-empirical system for the prediction of non-electrolyte activity in non-ideal mixtures. UNIFAC uses the functional groups present on the molecules that make up the liquid mixture to calculate activity coefficients. By using interactions for each of the functional groups present on the molecules, as well as some binary interaction coefficients, the activity of each of the solutions can be calculated. This information can be used to obtain information on liquid equilibria, which is useful in many thermodynamic calculations, such as chemical reactor design, and distillation calculations.