S. R. Bickham

Molecular dynamics simulations of shocked benzene

The behavior of benzene at high temperatures and pressures is studied using nonequilibrium molecular dynamics. The interatomic forces were generated using linear-scaling tight-binding electronic structure theory on systems containing 128 and 576 molecules. The shock Hugoniot, calculated directly from the simulations without predetermining the equation of state, is compared with experiment. Piston velocities of 4 km/s or greater result in a pressure-induced polymerization. This transition is consistent with the bend in the experimental measurements of shock versus piston velocity.

Molecular dynamics simulations of compressed hydrogen

Abstract 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). We find that the internal structure of warm, dense hydrogen has a pronounced time-dependent nature with the continual dissociation of molecules (dimers) and association of atoms (monomers). Finally, Hugoniots derived from the equations-of-state of these models do not exhibit the large compressions predicted by the recent laser experiments.

Tight-binding molecular dynamics of shock waves in hydrocarbons

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.

Simulations of shock-compressed hydrogen

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.