Molecular orbital ab initio calculations have been used to compute interact
ion energies between pairs of molecules in a large molecular cluster. These
energies are then used as the interaction energy parameters in the widely
used Wilson and UNIQUAC activity coefficient models. Low-pressure vapor-liq
uid equilibria predictions based on the calculated parameters have been com
puted for binary systems of water with methanol, ethanol, 1-propanol, 2-pro
panol, formic acid, acetic acid, acetone, acetonitrile, acetaldehyde, and m
-methylformamide. Excellent predictions are obtained with the UNIQUAC model
, whereas poor results are found with the Wilson model. In several cases, o
ur predictions are also superior to those obtained from UNIFAC. In addition
, using the same parameters and the UNIQUAC model, high-pressure vapor-liqu
id equilibria predictions were made using the Peng-Robinson-Stryjek-Vera eq
uation of state and the Wong-Sandler mixing rule for methanol, ethanol, 2-p
ropanol, and acetone separately with water. The low- and high-pressure resu
lts demonstrate that this unique approach can lead to accurate vapor-liquid
equilibrium predictions for hydrogen-bonding mixtures based only on pure-c
omponent properties and the structure of the molecules.