A three-body potential suitable for molecular dynamics (MD) simulations has been developed for vitreous silica by adding three-body interactions to the Born–Mayer–Huggins (BMH) pair potential. Previous MD simulations with the BMH potential have formed glassy SiO2 through the melt-quench method with some success. Though bond lengths were found to be in fair agreement with experiment, the distribution of tetrahedral angles was too broad and the model glass contained 6%–8% bond defects. This is indicative of a lack of the local order that is present in the laboratory glass. The nature of the short range order is expected to play an important role when investigating defect formation, surface reconstruction, or surface reactivities. An attempt has been made to increase the local order in the simulated glass by including a directional dependent term in the effective potential to model the partial covalency of the Si–O bond. The vitreous state obtained through MD simulation with this modified BMH potential shows an increase in the short range order with a narrow O–Si–O angle distribution peaked about the tetrahedral angle and a low concentration of bond defects, typically ∼1%–2%. The static structure factor S(q) is calculated and found to be in good agreement with neutron scattering results. Intermediate range order is also discussed in reference to the distribution of ring sizes.

A new interatomic potential for dissociative water was developed for use in molecular dynamics simulations. The simulations use a multibody potential, with both pair and three-body terms, and the Wolf summation method for the long-range Coulomb interactions. A major feature in the potential is the change in the shortrange O-H repulsive interaction as a function of temperature and/or pressure in order to reproduce the densitytemperature curve between 273 K and 373 at 1 atm, as well as high-pressure data at various temperatures. Using only the change in this one parameter, the simulations also reproduce room-temperature properties of water, such as the structure, cohesive energy, diffusion constant, and vibrational spectrum, as well as the liquid-vapor coexistence curve. Although the water molecules could dissociate, no dissociation is observed at room temperature. However, behavior of the hydronium ion was studied by introduction of an extra H + into a cluster of water molecules. Both Eigen and Zundel configurations, as well as more complex configurations, are observed in the migration of the hydronium.

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Molecular dynamics (MD) computer simulation of the adsorption of water molecules onto the vitreous silica surface was performed using a new dissociative water potential. 58 The simulations showed dissociative chemisorption of water molecules onto the silica surface, forming silanol (SiOH) groups at a concentration consistent with experimental data. Water penetration and silanol formation ∼7-8 Å below the outermost oxygen are observed. Because of the dissociative nature of the water potential, formation of hydronium ions is allowed, and, whereas seldom observed in the simulations of bulk water, hydronium ions are formed during the reactions causing the formation of the silanols. The formation of hydronium ions has also been observed in ab initio calculations of water adsorption onto silica surfaces. The time evolution of the reactions involving hydronium ions in our MD simulations is similar to that observed in first-principles MD calculations. Hydronium ions offer a mechanism by which initially singly coordinated terminal oxygen (Si-O -) receives a H + ion from a relatively distant chemisorbed H 2 O molecule via multiple H + ion transfer, creating two SiOH sites.

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