Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session H2: First Principles and Molecular Dynamics Calculations II |
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Chair: Ann Mattsson, Sandia National Laboratories Room: Fairmont Orchid Hotel Amphitheater |
Tuesday, June 26, 2007 1:45PM - 2:00PM |
H2.00001: Nonmetal-to-metal transition in warm dense hydrogen and helium Ronald Redmer, Bastian Holst, Andre Kietzmann, Nadine Nettelmann, Michael P. Desjarlais, Thomas R. Mattsson The precise knowledge of the equation of state of hydrogen and helium, especially at extreme conditions of pressure and temperature, is not only of fundamental interest but also necessary for models of interiors of giant planets such as Jupiter and Saturn. We have performed ab-initio quantum molecular dynamics (QMD) simulations for dense hydrogen and helium to study the thermophysical properties and the nonmetal-to-metal transition at high pressures. We present new results for the equation of state and the Hugoniot curves in the warm dense matter region. The optical conductivity is calculated via the Kubo-Greenwood formula from which the dc conductivity as well as the reflectivity are derived. We compare our results with shock wave experimental data as well as with other theoretical approaches. As a further application and test of the QMD equation of state data, the interiors of Jupiter and Saturn are modelled by solving the hydrostatic equation within a three-layer model. [Preview Abstract] |
Tuesday, June 26, 2007 2:00PM - 2:30PM |
H2.00002: Density Functional Theory in High Energy Density Physics: phase-diagram and electrical conductivity of water Invited Speaker: Atomistic simulations employing Density Functional Theory (DFT) have recently emerged as a powerful way of increasing our understanding of materials and processes in high energy density physics. \newline Knowledge of the properties of water (equation of state, electrical conductivity, diffusion, low-energy opacity) is essential for correctly describing the physics of giant planets as well as shock waves in water. Although a qualitative picture of water electrical conductivity has emerged, the necessary quantitative information is scarce over a wide range of temperature and density. Since experiments can only access certain areas of phase space, and often require modeling as a part of the analysis, Quantum Molecular Dynamics simulations play a vital role. \newline Using finite-temperature density functional theory (FT-DFT), we have investigated the structure and electronic conductivity of water across three phase transitions (molecular liquid/ ionic liquid/ superionic/ electronic liquid). The ionic contribution to the conduction is calculated from proton diffusion and the electronic contribution is calculated using the Kubo-Greenwood formula. The calculations are performed with VASP, a plane-wave pseudo-potential code. There is a rapid transition to ionic conduction at 2000 K and 2 g/cm$^3$, whereas electronic conduction dominates at temperatures at and above 6000 K\~[1]. Contrary to earlier results using the Car-Parrinello method\~[2], we predict that the fluid bordering the superionic phase is conducting above 4000 K and 100 GPa. Our comprehensive use of FT-DFT explains the new findings. The calculated conductivity is compared to experimental data. \newline I gratefully acknowledge Mike Desjarlais, my collaborator in this effort. The LDRD office at Sandia supported this work. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. \newline [1] T.R. Mattsson and M.P. Desjarlais, Phys. Rev. Lett. {\bf 97}, 017801 (2006). \newline [2] C. Cavazzoni, et. al. Science {\bf 283}, 44 (1999). [Preview Abstract] |
Tuesday, June 26, 2007 2:30PM - 2:45PM |
H2.00003: The relation between solitons and interactions in bcc materials Johannes Roth Shock waves in simple crystal structures are frequently accompanied by solitons, at least in molecular dynamics simulations. In this paper results for the case of bcc-materials shocked along a three-fold axis are reported. Pair potentials and generalized EAM potentials are studied. The influence of the Cauchy pressure is investigated by comparing iron and chromium. Solitons are observed for all interactions, but their strength and velocity varies. A modification of the Zhou-Wadley two- and three-body potential [Comp. Mat. Sci., in press] is introduced which permits a controlled variation of the Cauchy pressure in contrast to EAM. For this type of interaction the solitons are strongly depressed for negative Cauchy pressures. For the Dzugutov potential [PRB \textbf{72} (2005) 14126] a close relation between a subsonic phase transformation into an $\omega$-phase and supersonic solitons has been observed. A similar relation has not been found for iron and other interactions, although it might be favorable energetically. By ab-initio calulations with VASP it is shown that the $\omega$-phase transition in iron is surpassed by the transition into the hcp-phase. [Preview Abstract] |
Tuesday, June 26, 2007 2:45PM - 3:00PM |
H2.00004: Ab initio Molecular Dynamics Simulations of Water Under Shock Conditions N. Goldman, C.J. Mundy, I-F. W. Kuo, E.J. Reed, L.E. Fried, A. Curioni We report herein first principles simulations of water undergoing shock loading of velocities from 5 -- 11 km/s. Shocked aqueous solutions are of particular interest to understanding earth and planetary sciences, and the chemical reactivity that occurs within such hot, compressed systems. The Multi-Scale Shock Method (MSSM) utilizes a Lagrangian-derived constraint dynamics to restrict a molecular dynamics simulation to the thermodynamic states found in the shock. This allows for simulations with much smaller system sizes than previously required, and for significantly longer time scales than previously achievable. Consequently, for the first time, we have been able to conduct quantum simulations of a shocked material. We show that Density Functional Theory (DFT) molecular dynamics results compare extremely well to experiments on the water shock Hugoniot. We also present results for the ionic conductivity as well as the concentrations and lifetimes of chemical species found therein. Our results represent the strongest confirmation of the accuracy of DFT at high pressure and temperature that we know of, to date. [Preview Abstract] |
Tuesday, June 26, 2007 3:00PM - 3:15PM |
H2.00005: On Crystallization in Two-Component Quantum Coulomb Plasma Vladimir Filinov, Pavel Levashov, Vladimir Fortov, Michael Bonitz, Holger Fehske In this work we present the results of simulation of two-component fully-quantum Coulomb systems by Direct Path Integral Monte Carlo method. Our calculations show that at significantly high densities the fraction of bound states diminishes, and a crystallization can occur in the simulation box. This phenomenon exists at relatively low temperatures in the limited range of densites and at heavy-light particle mass ratio higher than 80 in 3D case. The crystal consists of heavy particles on the background of highly degenerate light particles; at certain conditions the heavy particle can form antiferromagnetic crystal-like structure. The crystal melts at temperature increase (the Lindemann criterion is valid in this case) and heavy-light particle mass ratio decrease (quantum melting). Our simulations generalize the earlier results for one-component plasma and can be applied to a number of problems: semiconductors under pressure, Coulomb crystals in white dwarfs and neutron stars, ion crystals in traps etc. Coulomb crystallization is also considered to be related to the high-temperature superconductivity. This work is supported by the CRDF and the Ministry of Education of Russian Federation Grant No.Y2-P-11-02. [Preview Abstract] |
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