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Theresults of a systematic investigation aimed at determiningthe dominant gas phase chemistry active during GaN MOVPE are reportedand discussed in this work. This study was performed developing athermodynamic database including the most stable GaN gas phase speciesand a gas phase mechanism that could efficiently describe their interconversionkinetics. The thermodynamic data and the kinetic mechanism were calculatedcombining density functional theory and ab initio simulations. Structuresand vibrational frequencies of reactants and transition states weredetermined at the M062X/6-311+G(d,p) level, whileenergies were computed at the ROCBS-QB3 level. Rate constants werecalculated using transition state theory using the rigid rotor - harmonicoscillator approximation and considering the possible degenerationof internal motions in torsional rotations. The thermodynamic analysisindicated that the Ga gas phase species formed in the highest concentrationat the standard GaN deposition temperature (1300 K) is GaNH2, followed by GaH and Ga. The diatomic GaN gas phase species, oftenconsidered to be the main precursor to the film growth, is predictedto be unstable with respect to GaNH2. Among the gas phasespecies containing two Ga atoms, the most stable are GaNHGaH(NH2)3, GaNHGaH2(NH2)2, and GaNHGa(NH2)4, thus indicating that thesubstitution of the methyl groups of the precursor with H or aminogroups is thermodynamically favored. Several kinetic routes leadingto the formation of these species were examined. It was found thatthe condensation of Ga(R1)x(R2)3–xspecies, with R1 and R2 being either CH3, NH2, or H, is a fast process, characterized by the formationof a precursor state whose decomposition to products requires overcomingsubmerged energy barriers. It is suggested that these species playa key role in the formation of the first GaN nuclei, whose successivegrowth leads to the formation of GaN powders. A kinetic analysis performedusing a fluid dynamic model allowed us to identify the main reactiveroutes of this complex system. [ABSTRACT FROM AUTHOR]