Cyclic cluster study of water adsorption structures on the MgO(1 0 0) surface

Cyclic cluster calculations were performed with the quantum chemical method MSINDO to elucidate the relative stabilities of c(4 × 2), p(3 × 2) and (1 × 1) overlayer structures of water molecules on the MgO(1 0 0) surface. For the c(4 × 2) and p(3 × 2) structures both molecular adsorption and partially dissociated adsorption were considered. In agreement with earlier theoretical studies partial dissociation was found to be more stable than molecular adsorption. For the c(4 × 2) structure both monolayer and double layer coverage were studied. Adsorption was found to be more stabilized with increasing degree of dissociation until 50% of the water molecules were dissociated. In the case of 50% dissociated water molecules we found that one quarter of the Mg atoms were pulled out of the MgO surface when surface relaxation was taken into account. A new structure for the fully dissociated (1 × 1) water monolayer was found which is considerably more stable than previously studied arrangements. In all cases surface relaxation was found to be important. The most stable structures of c(4 × 2), p(3 × 2) and (1 × 1) symmetry have adsorption energies which differ by no more than 13 kJ/mol. This offers an explanation of phase transitions of overlayer structures found by experiments between 180 and 300 K.