An important challenge in applied thermodynamics is the prediction of mixture phase behavior without the use of any experimental data. Current group contribution methods are sometimes, but not always successful in this regard. Activity coefficient models, such as UNIQUAC and Wilson, have explicit energy parameters that are usually fitted to experimental data. In this communication, we present a unique, new method of obtaining values for these parameters. We use ab initio quantum mechanics methods to obtain a solution of the Schrodinger equation and determine the molecular interaction energies resulting from hydrogen-bonding and van der Waals interactions. In particular, the Hartree-Fock (HF) method is used to determine the interaction energy between molecular pairs in clusters, and this information is then used in the UNIQUAC model to successfully predict the phase behavior. Examples given here are for the methanol + water, ethanol + water, acetic acid + water, and formic acid + water mixtures, the last of which is not accurately described by current group contribution methods. The analogous procedure using the Wilson model is less successful, suggesting that the energy parameters in the UNIQUAC model are of greater physical meaning.