previously he was at Sandia National Laboratories. His research concerns the energetics and kinetics of the reactions between minerals and aqueous solutions, and in recent work, he has determined rate coefficients for dissociation of Al−O bonds in various dissolved aluminum complexes.

The following paper presents the results of a theoretical study that probed the chemistry of water at structural defects on the MgO (001) surface. The computational technique used was periodic Hartree-Fock (PHF) theory with density functional based correlation corrections. The adsorption energies for water adsorbed on isolated comer, edge, and surface sites on the MgO surface were compared to the hydroxylation energies for the same sites. As stated in a previous paper, the binding of water to the perfect surface is exothermic by 4.1-5.6 kcal/mol whereas hydroxylating the perfect surface was endothermic by 24.5 kcal/mol. At step-edge sites, the process of water adsorption is exothermic and comparable in magnitude to the hydroxylation of these sites. The binding energies associated with water bound to the step-edge are 6.5-10.5 kcal/mol, and hydroxylation of this site is exothermic by 7.3 kcal/mol. At comer sites we find a strong preference for hydroxylation. The binding of water to a comer is exothermic by 20.7 kcal/mol, and hydroxylation is exothermic by 67.3 kcaymol. Mulliken populations indicate that the formation of a hydroxylated surface is governed by the stability of the hydroxyl bond where a hydrogen is bonded to a surface oxygen ion. As the coordination number of this oxygen binding site decreases, its ionic character also decreases, and it forms a more stable bond with the incoming hydrogen. This trend is confirmed by the densities of states for these sites. Finally, hydroxylation of the perfect (001) surface was examined as a function of lattice dilation. It was determined that, as the lattice constant increases, hydroxylation becomes more energetically favorable. This may be important in interpreting experimental thin-film results where the lattice constant of the substrate upon which the MgO film is deposited is slightly larger than that of bulk MgO.

A pairwise additive potential energy expression for the water/MgO interaction was obtained by fitting the parameters to ab initio electronic structure energy data, computed using correlation-corrected periodic HartreeFock (PHF) theory, at selected adsorbate/surface geometries. This potential energy expression was used in molecular dynamics and Monte Carlo simulations to elucidate the water/MgO interaction. Energy minimization reveals a nearly planar adsorbate/surface equilibrium geometry (-15°from the surface plane with the hydrogens pointing toward the surface oxygens) for an isolated water on perfect MgO (001), with a binding energy of 17.5 kcal/mol; subsequent PHF calculations on this system confirmed that this is a potential minimum. Rate constants for desorption (k dsorb ), intersite hopping (k hop ), intrasite rotation (k rot ), and intrasite flipping (k flip ) were estimated for an isolated water on the surface using simple transition state theory. The computed rates (at T ) 300 K) are k dsorb ) 1.1 × 10 5 s -1 , k hop ) 3.7 × 10 10 s -1 , k rot ) 5.7 × 10 11 s -1 , and k flip ) 4.6 × 10 11 s -1 . The motion of a single water on the surface is described by an effective diffusion constant (D eff ) 8.0 × 10 -6 cm 2 /s), computed from the surface rate constants combined with Monte Carlo simulations. The structure of the liquid water/MgO interface was determined from simulations with 64 and 128 water molecules on the surface. Simulations (at T ) 300 K) of the two-dimensional water overlayers reveal a densely packed first layer, Z(O w -surf) ) 2-3 Å, with one water per surface magnesium, with a nearly equal distribution of water molecules aligned -17°and +30°with respect to the surface plane. A more diffuse second layer exists, Z(O w -surf) ) 4-5.5 Å, with a much broader distribution of water angular orientations with respect to the surface plane. The region Z(O w -surf) > 6 Å resembles bulk water, with the density profile approaching a constant as a function of distance above the surface and a uniform distribution in water/surface angular orientations. At the water/vacuum interface (top of the multilayer) the waters assume a "planar orientation" (0°with respect to the surface plane). During the timescale of these simulations very little interlayer exchange of water molecules occurs between the first monolayer (n ) 1) and the additional overlayers (n g 2). In contrast, the water molecules in the multilayers (n g 2) display motion similar to bulk liquid water at this temperature.