Helmholtz Mixture Model
Helmholtz Mixture Model is the term used when the thermodynamic properties of a mixture are represented by a mole-fraction weighted combination of the generalized Helmholtz free energy equations of state for the pure fluids plus a residual function, as detailed by Lemmon and Tillner-Roth.
E. Lemmon and Tillner-Roth, R.
"A Helmholtz energy equation of state for calculating the thermodynamic properties of fluid mixtures"
Fluid Phase Equilibria 165, 1999. 1-21
Mixture equations of state are used in EES to provide property data for some R4xx fluids