MOLECULAR-DYNAMICS SIMULATIONS OF COMPRESSED LIQUID-HYDROGEN

Molecular dynamics simulations have been performed for highly compress ed fluid hydrogen in the density and temperature regime of recent shoc k-compression experiments. Both density functional and tight-binding e lectronic 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 dime r, of various crystalline structures, and of the fluid. The simulation s give pressures and electrical conductivities in general agreement wi th the measured values. The pressures are also compared with recent qu antum Monte Carlo results. This analysis provides a firm foundation fo r exploring the origins of the rapid change in electrical conductivity with temperature and density observed in the experiments. The simulat ions indicate that the conductivity in fluid hydrogen in this regime a rises both from: (1) closing of the band gap due to thermal effects an d compression; (2) electron hopping facilitated by the dissociated ato ms (monomers) with the latter process the most important. Finally, we find that the internal structure of cool, dense hydrogen has a pronoun ced time-dependent nature with molecules (dimers) constantly dissociat ing and atoms (monomers) constantly associating all of:he time. (C) 19 97 Published by Elsevier Science Ltd. All rights reserved.