Cluster calculations employing hybrid density functional theory have been carried out to examine the atomistic details and thermochemistry of the early stages of Al2O3 atomic layer deposition (ALD) on the Si(100)-2 x 1 surface using the gas-phase precursors, trimethylaluminum (TMA) and H2O. The critical point structures and enthalpies characterizing both the Al- and O-deposition half-reactions were investigated. Both sets of ALD half-reactions were found to be thermodynamically favorable and kinetically uninhibited. For all reactions the transition states and reaction products were determined to be lower in energy than the starting reactants. The H2O ALD half-reactions were found to have an overall reaction enthalpy between -1.45 and -1.63 eV, with a transition state energy of -0.13 to -0.21 eV. The TMA ALD half-reactions were found to be exothermic by 1.85-1.88 eV. The transition state energy for the Al deposition half-reactions were determined to be -0.19 to -0.27 eV. Careful comparison of the reaction enthalpies suggests a small reactivity dependence on neighboring -OH* groups, activating and suppressing the Al- and O-deposition ALD half-reactions, respectively.