Modeling of organometallic vapor phase epitaxy requires computation of Gibbs free energies and reaction rate constants associated with chemical reactions that may occur in the gas phase or with the species adsorbed to a substrate. These parameters are to be used in the simulation of group III-group V epitaxy in high-pressure chemical vapor-deposition reactors, under different conditions of pressure, temperature, and flow rates. A computational fluid-dynamic program is used to solve for time-dependent or steady-state gas-phase and heterogeneous chemical kinetic equations coupled with fluid dynamic equations. In this talk, the calculations of the required chemical parameters are discussed. These parameters are obtained from molecular orbital calculations and semi-classical transition-state theory. In particular, the interfacial reactions are studied using the hybrid approach ONIOM, which divides the objects into layers that are calculated using different computational methods. The important reactions considered are those that involve the dissociation of the source materials, the adsorption of the source species onto the substrate or the growing film, and the reactions between different source species in the gas phase as well as adsorbed. Preliminary results are presented.