Computational quantum mechanics is leading to new, theoretically based methods for the prediction of thermodynamic properties and phase behavior of interest to engineers. Three such methods we have been working on are reviewed here. In the most direct and computational intensive form, computational quantum mechanics is used to obtain information on the multidimensional potential energy surface between molecules, which is then used in computer simulation to predict thermodynamic properties and phase equilibria. At present, this method is limited to the study of small molecules due to the computational resources available. The second method is much less computationally intensive and provides a way to improve group-contribution methods by introducing corrections based on the charge and dipole moment of each functional group that is unique to the molecule in which it appears. The final method we consider is based on the polarizable continuum model, in which the free energy of transferring a molecule from an ideal gas to a liquid solution is computed, leading directly to values of activity coefficients and phase equilibrium calculations. (C) 2003 Elsevier Science B.V All rights reserved.