H2O Dissociation at Low-Coordinated Sites on (MgO)n Clusters, n = 4, 8

We investigate the interactions of one and two H 2 O molecules at three-coordinated sites of (MgO) 4 and (MgO) 8 clusters. Energies of adsorption are calculated and compared to similar work involving periodic and embedded cluster systems. The cluster model results predict that H 2 O chemidissociates without a barrier directly onto adjacent three-coordinated sites. The chemidissociation energy per water molecule is observed to be greater upon the dissociation of two neighboring water molecules compared to the dissociation of a single water molecule: a positive cooperativity effect. Correlation energy treated at the MP2 level of theory reduces the predicted chemidissociation energy by approximately 10 kcal/mol, and it is found that the MP2 single-point energies evaluated at the RHF-optimized geometries, MP2//RHF, differ from the energy of the MP2-optimized structures by approximately 1 kcal/mol.