USE OF ABINITO-QUANTUM MECHANICS CALCULATIONS IN GROUP CONTRIBUTION METHODS .2. TEST OF NEW GROUPS IN UNIFAC

In this paper we show that functional groups identified previously from ab initio molecular orbital calculations lead to better UNIFAC predictions of vapor-liquid equilibrium, especially for mixtures involving molecules in which proximity effects are important. We also find that, for the functional groups we have defined, in many cases it is generally not necessary to distinguish between subgroups. This reduces the total number of main groups and subgroups that must be considered. For some hydrogen-bonding mixtures, neither the new nor the old functional groups lead to satisfactory excess Gibbs free energy predictions. This is in agreement with our previously reported supermolecule calculations which showed that hydrogen-bonding energies did not satisfy group additivity and therefore could not be accounted in group contribution methods.