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The warm, dense matter (WDM) regime requires a sophisticated treatment, since neither ideal gas laws or fully ionized plasma models apply. Mixtures represent the predominant form of matter throughout the universe and the ability to predict the properties of a mixture, through direct simulation or from convolution of the properties of the constituents is both a challenging prospect and an important goal. Through quantum molecular dynamics, we accurately simulate WDM and compute equations of state, transport, and optical properties of such materials, including mixtures, in a self-consistent manner from a single simulation. With the ability to directly compute the mixture properties, we are able to validate mixing rules for combining the optical and dynamical properties of Li and H separately to predict the properties of lithium hydride (LiH). We have examined two such mixing rules and extend them to morphologies beyond a simple liquid alloy. We have also studied a mixture of polyethylene and aluminum at T = 1 eV. [ABSTRACT FROM AUTHOR]

In the plastic bonded explosive PBX 9501, a 50/50 mixture of Estane 5703 (a polyester urethane random copolymer) and nitroplasticizer (NP) binds the HMX explosive crystals. Chemical kinetic mechanisms are being developed for the thermal degradation of NP for high temperatures (explosions) and low temperatures (natural and accelerated aging studies). The goal of this work is to investigate reaction mechanisms using density functional electronic structure theory in addition to the data obtained from explosion and aging experiments. Very little is known of the mechanisms for the oxidation of polymers by oxidants other than molecular oxygen. As the first step in the aging of PBX 9501 in a low moisture and low oxygen environment, we propose that NP decomposes into oxidizing gases (NO2 and NO). © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We show that Quantum Molecular Dynamics provides a powerful tool to extend and benchmark current opacity libraries into the complex regime of warm dense matter. In this regime, the medium can be constituted of electrons, protons, atoms and molecules, while plasma and many body effects can not be treated as perturbations. Among the most notable features of this new approach for calculating Rosseland mean opacities is the ability to obtain a consistent set of material, optical and electrical properties for various mixtures from the same simulation. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The physics of sliding at compressed Ta/Al interfaces is discussed based upon the results of large scale 3D NEMD simulations. A new set of Embedded Atom Method potentials has been constructed to treat the Ta-Al interaction. Pressures of order 15 GPa are studied and the velocity dependence of the frictional force is studied for several interfacial configurations including Al(100)/Ta(100) and Al(111)/Ta(110). Generic behavior is observed, characterized by a linear increase at low velocities followed by a power law decrease at high velocities associated with near interface structural transformation in Al. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We performed molecular dynamics simulations to obtain the internal energy and pressure of shock-compressed fluid nitrogen. Our calculations were performed using the generalized gradient approximation (GGA) in density functional theory. From this equation of state, we derived the principal and second shock Hugoniots in the density region probed by gas-gun experiments. While the single-shock Hugoniot derived from this equation of state agrees well with gas-gun experiments; in contrast, the second-shock Hugoniots show discrepancies with the experimental measurements. This is particularly the case in the region where negative Grüneisen parameters were deduced experimentally and where shock cooling was measured. [ABSTRACT FROM AUTHOR]

The behavior of shock-compressed methane, benzene, and polyethylene at high temperatures and pressures is studied using non-equilibrium molecular dynamics and linear-scaling tight-binding electronic structure theory. The quantum mechanical tight-binding description provides a reasonable model to study the dynamics of dissociating hydrocarbons (making and breaking of chemical bonds) at high temperatures and pressures. The shock Hugoniots, calculated directly from the simulations without predetermining the equation of state, are compared with experiment. For certain piston velocities, a chemical dissociation wave evolves behind the compressive shock front. For all three hydrocarbons considered, the dissociation region contains molecular hydrogen formed from atomic hydrogen released from broken C-H bonds. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations have been performed for highly compressed fluid hydrogen in the density and temperature regime of recent shock-compression experiments. Both density functional and tight-binding electronic structure techniques have been used to describe interatomic forces. Two tight-binding models of hydrogen have been developed with a single s-type orbital on each atom that reproduce properties of the dimer, of various crystalline structures, and of the fluid. The simulations indicate that the rapid rise in the electrical conductivity observed in the gas-gun experiments depends critically on the dissociated atoms(monomers). Hugoniots derived from the equations-of-state of these models do not exhibit the large compressions predicted by the recent laser experiments. [ABSTRACT FROM AUTHOR]