The theory obtains the excess compressibility factor of the mixture by scaling the excess compressibility factor of a reference mixture of hard spheres. Spheres of species i in the reference mixture must have a molecular volume equal to the effective molecular volume of species i in the true mixture. This effective molecular volume is introduced to account for the fact that the excluded volume of a nonconvex molecule is greater than its real volume. The theory requires two parameters, namely the effective molecular volume of the mixture and the scaling parameter. Both can be derived from the geometry of the molecules which form the mixture.
Results are in good agreement with simulation data.