A multiscale simulation approach, including atomistic quantum mechanical calculations on the molecular scale and physical continuum mechanical process simulations on the macroscopic scale, has been used to study the initial gas-phase decomposition of a single-molecule precursor. The compound bisazido(dimethylaminopropyl)gallium (BAZIGA), that forms gallium nitride in the absence of other nitrogen sources, is proposed to decompose by a multistep reaction mechanism to the key intermediate, GaN3. The reaction rates for the effective overall reaction were derived on the basis of transition state theory in an ab initio fashion from the results of density functional theory-type calculations. Methods to accurately approximate the entropic contributions to the free energies of activation for the elementary steps were developed and employed. The calculated rates were fit to an Arrhenius type rate law with an apparent energy of activation of Ea = 321 kJ mol -1 (A=9.90× 1020 s-1). Process simulations were performed for a real vertical stagnation flow test reactor. The inclusion of radiative heat transfer was found to be crucial to reproduce the experimentally determined temperature distribution on the reactor wall. By inclusion of the gas phase decomposition to GaN3 in terms of an effective one-step reaction, only the intermediate GaN3 was found to contribute to film growth. It is concluded that all parent precursor molecules decompose in the hot zone above the substrate and do not reach the surface under the given conditions.