Semiempirical mixing rules

Modified VDW one-fluid mixing rules. Semiempirical mixing rules. 

It is difficult to ascertain the reasons for the magnitude of improvement in vapor-liquid predictions using the semiempirical mixing rules instead of the modified VDW one-fluid rules. The semiempirical rules probably offset the truncation error in the approximation A = Ax to some extent. It certainly is interesting that the orientation parameter yx = 8/ is independent of the set of characteristic energy parameters, cy, for the mixture. This possibly indicates for normal paraffin hydrocarbons (and perhaps similar nonpolar fluids) that contributions of molecular attraction and orientation effects are essentially independent, but that steric and size effects are dependent. 

VDW one-fluid rules. Comparisons of predicted and experimental vapor-liquid equilibrium for ternary and multicomponent systems are given in Tables V, VI, and VII, for both the semiempirical and VDW one-fluid mixing rules. In these calculations, the unlike interaction parameters for interactions of ethane and heavier components with each other were taken to be unity. This is a reasonable approximation for the unlike interaction parameters for the heavier components for the interaction of ethane and... 

This restriction rules out all discrete models exclusively based on semiempirical force fields, leaving among the discrete models the MC/QM and the MD/QM procedures, in which the second part of the acronyms indicates that the solute is described at the quantum mechanical (QM) level, as well as the full ab initio MD description, and some mixed procedures that derive the position of some solvent molecules from semiclassical simulations, replace the semiclassical description with the QM one, and repeat the calculation on these small supermolecular clusters. The final stage is to perform an average on the results obtained with these clusters. These methods can be used also to describe electronic excitation processes, but at present, their use is limited to simple cases, such as vertical excitations of organic molecules of small or moderate size. This limitation is due to the cost of computations, and there is a progressive trend toward calculations for larger systems. 

---