We have used Hartree-Fock theory and density functional theory to compute the enthalpy and entropy changes of dimerization for water, methanol, and the family of carboxylic acids. These results are used in a physical equation of state, the statistical associating fluid theory (SAFT), in order to model the phase behavior of these hydrogen-bonding compounds. A procedure has been developed to relate the calculated enthalpy and entropy changes to the association parameters in SAFT using only low-pressure data, as well as to relate molar volumes from molecular orbital calculations to the segment size and chain length parameters in SAFT. By doing so, the SAFT model is reduced to a three-parameter equation of state for associating fluids. The modified equation of state is shown to be as accurate as the original SAFT model for correlating pure-component vapor-liquid equilibrium data with fewer adjustable parameters.