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In molecular simulation research powerful computers are used to accurately model real chemical systems using statistical mechanics and quantum mechanics. This provides a sort of "virtual laboratory" in which almost any property can be measured or examined, including those which are not accessible to experiments. We use simulations to develop an atomic-scale understanding of the behavior of complex systems. Our current research includes:
First-principles Monte Carlo simulations of phase equilibria at extreme conditions. By coupling widely-used Density Functional Theory electronic structure codes to advanced Monte Carlo simulation algorithms, we can obtain liquid-vapor and liquid-liquid coexistence lines at extremes of temperature and pressure not easily accessed by experiment. We are currently working on phase equilibria of atomic systems, such as lithium metal, lithium/sodium mixtures, phosphorous, sulfur, and carbon.
Multiscale simulations of sol-gel materials. Xerogels and aerogels are nanostructured porous materials prepared using sol-gel processes that can display a wide range of chemical, topological and morphological characteristics. They are extensively used in chromatography, catalysis, optics, biotechnology and chemical sensing. The relationship between the complex structures of these materials and their useful properties is poorly understood. We address this by developing realistic models of these materials through a multi-scale approach involving: (1) quantum mechanics, used to understand the interactions between gel precursor species and to parameterize reactive simulation models, (2) molecular simulations, to model the growth and structure of small silica aggregates in solution (the "sol"), and (3) coarse-grained and lattice-type models, to study the large-scale evolution of gel structures and solvent-induced structural changes during gel drying.
Figure 1: Coarse-grained aerogel model. The color scheme indicates connectivity, with red particles having only one bond, and purple ones having at least five.
Capillary phenomena. The behavior of gases and liquids in nanometer-scale pores is considerably different than in the bulk, and is important in separation technologies, catalytic reactors, lubrication, nano-scale devices, and materials characterization. Using recently developed simulation methods, we can directly probe the dynamics and thermodynamics of these systems.
Figure 2: Capillary condensation in a simple aerogel model. Low pressure is shown at left, and higher pressure at right. The colors indicate fluid density on a "temperature scale", with blue representing vapor and red representing liquid.
We are also interested in the development of new simulation methodologies, high-performance computing and parallel simulation algorithms, and computer graphics visualizations; in this area we are participating in an NSF-supported collaborative project entitled "Cyberinfrastructure for Phase-space Mapping: Free Energies, Phase Equilibria and Transition Paths" Our specific projects in this area include studies of clathrate hydrates confined in porous materials, and simple models of multicomponent fluids, one of which is shown in Figure 3.
Figure 3: Binary mixtures of "tetrominoes," a simple model in which localized aggregation of like-shaped objects is driven purely by entropy.
