I. Kwon

Molecular dynamics simulations of compressed liquid 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. 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 the time.

Quantum molecular dynamics simulations of dense matter

Quantum molecular dynamics simulations of pure samples and of mixtures of various species yield important structural, dynamical, and electronic properties that characterize matter at high compressions and moderate temperatures. A fully quantum mechanical treatment of the electrons, contained in a periodically replicated reference cell of Na atoms, by density functional and semi-empirical methods provides an accurate representation of the force on the nuclei and of the electronic structure of the medium. The nuclei move according to the classical equations of motion in response to this quantal force. Detailed comparisons of the models with other calculations are presented together with a comprehensive description of the techniques.

Simulation of impurity line shapes in a hot, dense plasma

Abstract The widths and positions of spectroscopic lines of neutral, hydrogen- and heliumlike argon impurities in a dense, hot hydrogen plasma are reported. The calculations employed an integrated two-step model in which the atomic positions evolved through a molecular dynamics (MD) simulation using several different effective pair potentials and the electronic structure arose from calculations on an ArH n cluster. Integrating the resulting level shifts over an MD trajectory produced the desired line profile. We compare against other calculations in the realm of 1000 eV temperature and densities 0.25–8 g/cm 3 and present results for low temperature (10 eV).

Molecular dynamics simulations of dense plasmas

We have performed quantum molecular dynamics simulations of hot, dense plasmas of hydrogen over a range of temperatures (0.1–5 eV) and densities (0.0625–5 g/cc). We determine the forces quantum mechanically from density functional, extended Huckel, and tight binding techniques and move the nuclei according to the classical equations of motion. We determine pair‐correlation functions, diffusion coefficients, and electrical conductivities. We find that many‐body effects predominate in this regime. We begin to obtain agreement with the OCP and Thomas‐Fermi models only at the higher temperatures and densities.

Quantum-Molecular-Dynamics Simulations of Isotopic Mixtures of Dense, Hot Hydrogen

Calculations of isotopic binary mixtures of hydrogen over a range of densities and temperatures were performed using quantum-molecular-dynamics simulations. The electronic component was treated with three different models including density functional (local density approximation), tight binding, and effective pair potentials. Quantum-mechanical effects were found to be important even at elevated temperatures. Self-diffusion coefficients for each component became comparable in magnitude with increasing density in contradiction to simple mass-scaling rules. Mutual-diffusion coefficients tended to exceed the simple concentration average of the self-diffusion coefficients, but by less than 25%.

Electrical conductivity of compressed argon

The authors report calculations of the electrical conductivity of solid argon as a function of compression within the density functional local density approximation formulation for a norm-conserving pseudopotential using both electron-phonon coupling and molecular dynamics techniques.