Molecular dynamics simulations of compressed liquid 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. A new tight-binding model of hydrogen has been developed with a single s orbital on each atom that reproduces properties of the dimer, of various crystalline structures, and of the fluid. The simulations give pressures and electrical conductivities in general agreement with the measured values. The pressures are also compared with recent quantum Monte Carlo results. This analysis provides a firm foundation for exploring the origins of the rapid change in electrical conductivity with temperature and density observed in the experiments. The simulations indicate that the conductivity in fluid hydrogen in this regime arises both from: (1) closing of the band gap due to thermal effects and compression; (2) electron hopping facilitated by the dissociated atoms (monomers) with the latter process the most important. Finally, we find that the internal structure of cool, dense hydrogen has a pronounced time-dependent nature with molecules (dimers) constantly dissociating and atoms (monomers) constantly associating all of:he time. (C) 1997 Published by Elsevier Science Ltd. All rights reserved.