A new empirical pairwise potential model for ionic and semi-ionic oxides has been developed. Its transferability and reliability have been demonstrated by testing the potentials toward the prediction of structural and mechanical properties of a wide range of silicates of technological and geological importance. The partial ionic charge model with a Morse function is used, and it allows the modeling of the quenching of melts, silicate glasses, and inorganic crystals at high-pressure and high-temperature conditions. The results obtained by molecular dynamics and free energy calculations are discussed in relation to the prediction of structural and mechanical properties of a series of soda lime silicate glasses.

Most molecular theories of nematic liquid crystals assume that the constituent molecules are cylindrically symmetric. However, although this may be a useful approximation the molecules of real nematogens are of lower symmetry ; here we develop a theory for an ensemble of such particles based on a general expansion of the pairwise intermolecular potential together with the molecular field approximation. The dependence of the orientational properties of the uniaxial mesophase on the deviation from molecular cylindrical symmetry is calculated from the series expansio n of the pseudopotential. In these calculations the number of arbitrary parameters in the orientational pseudo-potential is reduced by assuming that the anisotropic intermolecular potential originates solely from dispersion forces. The theoretical predictions for the values of the ordering matrix and the entropy change at the nematic-isotropic transition are found to be in good agreement with those observed for 4,4'-dimethoxyazoxybenzene. In addition, the theory provides a reasonable account of the temperature dependence of the order parameter at constant volume for this nematogen. The allowance for deviations from molecular cylindrical symmetry appears to remove many of the discrepancies between the Maier-Saupe theory and experiment.

Molecular dynamics simulations and energy-minimization techniques have been applied for the first time to determine the whole set of elastic properties (Young's modulus, shear modulus, bulk modulus, and Poisson's ratio) of alkali silicate glasses with different ion modifiers (Li, Na, and K) in the range 0-30 mol % alkaline oxide. Excellent agreement has been found between the simulation results and the experimental data. The peculiar behavior of the Li-containing glasses with respect to the Na and K ones is extensively discussed in terms of the glass structural features. It is found that the elastic property variation as a function of alkali addition can be explained by three concurrent factors: (1) depolymerization of the silica network; (2) increasing the cohesion of the glass by the establishment of alkali-NBO bonds; and (3) decreasing the free volume with consequent increasing of the glass packing density.