ONIOM calculations using CCSD theory have been performed to study the dissociation of hydrogen and water molecules on ZnO surface. Three different cluster models were used to study the influence of ONIOM layer size at the interaction energies and optimized geometries. The geometric results indicate a near tetrahedral geometry for the interaction of zinc and oxygen sites of dissociated water. The interaction energies are larger for the water dissociation. The ONIOM layer size has more influence on the interaction energies when compared with the geometric parameters and mainly for water dissociation. (c) 2012 Wiley Periodicals, Inc.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)