Cluster based density functional calculations have been carried out to examine the effect of steric interactions on chemical reactions at the H/Si(111) surface, adopting the initial Al2O3 atomic layer deposition (ALD) surface reactions involving H2O and Al(CH3)(3) (TMA) as our test reactions. Results obtained for the minimal Si10H16 cluster and two larger Si43H46 ONIOM embedded clusters reveal a substantial difference of up to 0.28 eV (6 kcal/mol) in the predicted activation energies involving TMA, revealing the inadequacy of small clusters to model atomically dense semiconductor surfaces.