Industrial equations of state have similar mixing and combining rules. These include the van der Waals one-fluid mixing rules  and the van der Waals combining rule, which introduces a binary interaction parameter formula_23,  There is also the Huron-Vidal mixing rule, and the more complex Wong-Sandler mixing rule, which equates excess Helmholtz free energy at infinite pressure between an equation of state and an activity coefficient model (and thus with liquid excess Gibbs free energy).