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Stephane Mazevet T-4 Lee A. Collins T-4 Joel D. Kress T-12 |
We
have established over the past several years that quantum molecular
dynamics (QMD) simulations accurately capture the intricate nature of
matter under High Energy-Density (HEDP) and warm dense matter (WDM)
conditions. This regime spans pressures from a few to a few hundreds GPa
(few Mbar) and temperatures from about one to several Electron-Volts
(eV). In the laboratory, this regime is reached by shock compression at
experimental facilities such the Z-pinch at Sandia, the two-stage
gas-gun at LLNL, and laser experiments such as NOVA at LLNL, OMEGA at
Rochester, and in the future at the NIF facility  currently under construction at LLNL. Since
experiments have proven difficult in many important regimes, as
witnessed by the controversy over the equation-of-state (EOS) of
compressed deuterium, MD simulation techniques provide one of the best
venues for making meaningful critiques of physical models that account
for pressure ionization, recombination, molecular dissociation and
association of atomic and molecular fluids as well as molecular mixtures
in atomic modeling codes. Using the Kubo-Greenwood formulation, the
simulations yield a consistent set of dynamical, electrical, and optical
properties such as the dielectric function, reflectivity, and
absorption coefficients as well as light element Rosseland mean
opacities for various atomic and molecular mixtures. To address the new challenges posed by emerging experimental capabilities (i.e., NIF, Z-pinch), we are currently extending the current method to address non-equilibrium phenonema induced by laser-matter interaction, laser-induced shocks and phase transitions, as well as the dynamical response of solids. The short term goals are to determine consistent optical and transport properties (with and without magnetic fields) for materials of relevance to the Laboratory mission and astrophysical applications. The latter requires significant innovations to include the latest developments in Density Functional Theory methods (Hybrid functionals and time dependent methods) while the former necessitates the inclusion of non-adiabatic effects in the simulations. |
Examples of HEDP/WDM Phenomena Studied
| Dynamical Properties-Equation of State
When shocked up to 100GPa (1Mbar), molecular liquids such as H, N, O, NO dissociate. The QMD simulations performed describe from an ab-initio standpoint the EOS and Hugoniots in this pressure- temperature region. Remarkable agreement is obtained for various species with experimental measurements performed on the Z-pinch at Sandia, and the 2 stage Gas Gun at LLNL. A notable exception is the discrepancy between the QMD Hydrogen EOS and the NOVA measurements.
QMD simulations follow the constituency of the molecular fluid along shock Hugoniots in the delicate pressure region where various chemical processes such as molecular dissociation and association, ionization and recombination compete. The figure below shows the chemical evolution of shocked nitrogen oxide (NO) as evidenced by the pair distribution functions, g(r), of the various diatomic species involved.
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Non-metal metal transition induced by pressure As materials are shocked and their
constituency evolves, their physical properties change dramatically
compared to ambient conditions. Over the past few years, a particular
fertile area of research has centered on establishing the pressure-
temperature conditions at which various insulating materials exhibit a
metallic behavior. Using QMD simulations, we produced accurate
electrical properties of H, N(figure shown below), O, He, H-He mixtures,
Al,.... for various density-  The figure below shows a comparison between the QMD results and experiment (performed at OMEGA) for the reflectivity of silica, one of the best insulators known, up to 14Mbar.
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The description of various stellar objects such as white dwarfs, requires an accurate knowledge of the optical properties of hydrogen, helium and hydrogen-helium mixtures in the difficult regime of low temperature (few eV) and high density (several g/cm^3). In this regime, the environment can no longer be treated as a perturbation and standard opacity calculations usually fail. Using QMD simulations, we calculate optical properties consistent with the equation of state and provide benchmarks for standard opacity calculations. As a representative example, we show below the absorption coefficient of aluminum at constant temperature and increasing pressure as it evolves from an atomic gas to a degenerate plasma due to "pressure ionization".
The figure below shows a comparison between a QMD calculation of the hydrogen Rosseland mean opacity at constant temperature and increasing pressure and the Los Alamos Light Element opacity table LEDCOP.  |
S. Mazevet, M.P. Desjarlais, L.A. Collins, J.D. Kress, N.H. Magee, "Simulations of the Optical Properties of Warm Dense Aluminum", submitted to Phys. Rev. E (2004).Representative Publications
S. Mazevet, P. Blottiau, J.D. Kress, and L.A. Collins, "Quantum Molecular Dynamics Simulations of shocked Nitrogen Oxide", Phys. Rev. B 69, 224207 (2004).
Yann Laudernet, Jean Clerouin, and Stephane Mazevet , "ab initio simulations of the electrical and optical properties of shock compressed SiO2", Phys. Rev. B (2004).
S. Mazevet, L. A. Collins, N.H. Magee, J. D. Kress, and J.J. Keady, "Quantum Molecular Dynamics calculations of radiative opacities", Astronomy & Astrophysics 405, L5 (2003).
S. Mazevet, J. D. Kress, and L. A. Collins, "Quantum molecular-dynamics study of the electrical and optical properties of shocked liquid nitrogen", Phys. Rev. B; 67, 4201 (2003).
S. Mazevet, J. D. Johnson, J. D. Kress, L. A. Collins, and P. Blottiau, ``Density Functional Calculation of Multiple-shock Hugoniots of Liquid Nitrogen'', Phys. Rev. B 65, 014204 (2002).
M.P. Desjarlais, J.D. Kress, L.A. Collins, "Electrical conductivity for warm, dense aluminum plasmas and liquids", .Phys. Rev. E 66, 025401 (2002).
L.A. Collins, S.R. Bickman, J.D. Kress, S. Mazevet, T.J. Lenosky, N.J. Troullier, W. Windl,``Dynamical and Optical Properties of Warm, Dense Hydrogen'', Phys. Rev. B 63, 4110 (2001).
B. Militzer, J. D. Johnson, L. A. Collins, S. Mazevet, J.D. Kress, and D. Cepperley, "Calculations of the Deuterium Double Shock Hugoniot from Ab-initio Simulations", Phys. Rev. Lett. 87, 275502 (2001).
T.J. Lenosky; S.R. Bickham; J.D. Kress; L.A. Collins, "Density-functional calculation of the Hugoniot of shocked liquid deuterium", Phys. Rev. B ; 61 p1 (2000).
