Computational studies of model glass-forming systems

This dissertation presents a collection of computational studies of model supercooled and glass-forming systems. In Chapter 2, we present a family of systematically softened potentials based on the well-known Lennard-Jones potential, and investigate how perturbations to the repulsive exponent can affect the thermodynamic and dynamic properties of a system. The softer liquids have markedly higher entropies and lower Kauzmann temperatures than their Lennard-Jones counterparts, and remain diffusive down to appreciably lower temperatures. Chapter 3 provides a critical analysis of the validity of using a relaxation time as a substitute for viscosity when studying Stokes-Einstein behavior in simulations. For both model atomic (family of softened potentials) and molecular (Lewis and Wahnstrom model of ortho-terphenyl) systems, the validity of this substitution, and assumption of the interchangeability of different relaxations times, are strongly challenged. Chapter 4 contains a comprehensive study of the viscosity of the SPC/E model of water. We map the anomalous region for viscosity (decrease of viscosity upon compression) on the (rho, T) plane, and extend the discussion of Chapter 3 by applying a similar analysis to water. We also present two studies of properties of free-standing films. Films composed of the binary Lennard-Jones glass-forming mixture (Chapter 5) exhibit substantial compositional inhomogeneity, while films composed of rigid Lewis and Wahnstrom ortho-terphenyl molecules (Chapter 6) show oscillatory orientational preferences induced by the surface. In all cases, diffusivity at the surface is greatly enhanced relative to the interior. Additionally, we perform an energy landscape analysis of these films, and find that molecules at the surface are able to sample the underlying energy landscape more effectively than those in the interior.