Atomistic modeling of reconstructions and relaxation of the nonpolar (<10(1)over bar 0>) and (<11(2)over bar 0>) ZnO surfaces has been performed to obtain more realistic surface structures for embedded cluster quantum chemical calculations of the reactivity of ZnO toward H-2 dissociation. Only small differential geometrical effects are found for the cation and anion top-layer relaxations. Both surfaces are found to be unreactive toward H-2 dissociation. For the polar, Zn-terminated (0001) surface several different reconstructions were studied; the lowest surface energy was obtained fur a 4x4 reconstruction containing single-step plateaus and valleys. The highest exothermicity was found for H-2 dissociation over a corner site, involving a low-coordination oxygen, on the plateau. The edge sites on the plateau also lead to exothermic reactions, while the regular sites between plateaus are unreactive. The determining factor for the exothermicity is the coordination of the anion, but the ionization potential at the anion site also shows a correlation with the exothermicity. No cooperative effects were found. The vibrational frequencies were computed in qualitative agreement with experiment for the O-H vibration, while the strong experimental Zn-H peak at 1710 cm(-1) has not been reproduced. The quantum chemically computed chargeson the ions are found to vary with coordination and position; this could have an effect on the atomistic modeling which presently does not include coordination-dependent terms in the potentials. Using an unembedded, gas-phase ZnO unit as a model leads to unrealistically high reaction energies; this is also the case if reconstruction of the (0001) surface is neglected. This demonstrates the need to develop realistic embedding schemes.