Where, in the grand canonical ensemble, the partial derivative ∂U a ∂n a i is evaluated using the ensemble fluctuations in the energy of the system and the number of particles as shown in the work of Vlugt et al. , RT is the ideal gas term, and (h * b -h b ) i is the departure function for the partial molar enthalpy in the bulk fluid. In simpler terms, the last term accounts for non-ideal behavior. For more details on the derivation of equation 1, please see the work of Karavias and Myers 21 .