By default, for fixed composition, variable composition, and reacting mixtures, all mixture properties use the Ideal Mixture option. With this option, a mixture property (or its inverse, for the cases of density and molar mass) is computed as a mass-weighted average of the component properties (or their inverses, for the cases of density and molar mass).

In order to set options other than Ideal Mixture for the Thermodynamic Properties, select Mixture Properties in the Mixture Properties tab in the Material details view and set Option to Non Ideal Mixture. When the global option is set to Non Ideal Mixture, individual properties may still be set to use the ideal mixture rule by selecting Ideal Mixture for the respective property.