This method, based on the UNIFAC expression, was developed for modeling polymer systems. The liquid activity coefficient is computed as a combination of the same combinatorial and residual terms used in the original UNIFAC equation plus a free volume effect term.
The free volume effect term is computed by using the molecular weights, mass densities and weight fractions for the components, as well as a constant to represent the effective degrees of freedom per molecule of solvent.
Refer "UNFV - Free volume modification to UNIFAC" under "Liquid Activity Coefficient Methods" in Chapter 2, Volume 1 of the PRO/II Reference Manual for more information
Pure Species Parameters for Polymers