where φj is the volume fraction of component j. This rule has been used in several surrogate studies . Since ρ is a thermodynamic property, it can also be calculated with equations of state (EoSs) . EoS calculations are more precise and can estimate ρ at variable temperature and pressure, but they also require extensive parameters for each species, which limits the range of molecules that can be considered. For example, the National Institute of Standards and Technology (NIST) REFPROP software supports less than 130 hydrocarbons and is limited to mixtures with 20 or fewer components .