Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session B5: First-Principles & Molecular Dynamics Calculations I |
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Chair: Stephane Mazevet, Los Alamos National Laboratory Room: Hyatt Regency Constellation F |
Monday, August 1, 2005 9:00AM - 9:15AM |
B5.00001: Designing Meaningful Density Functional Theory Calculations in Materials Science A.E. Mattsson, P.A. Schultz, M.P. Desjarlais, T.R. Mattsson, K. Leung Density functional theory (DFT) methods for calculating the quantum mechanical ground states of condensed matter systems are now a common and significant component of materials research. These methods are also increasingly used in Equation of State work, in particular in the warm dense matter regime. The growing importance of DFT reflects the development of sufficiently accurate functionals, efficient algorithms, and continuing improvements in computing capabilities. As the materials problems to which DFT is applied have become large and complex, so have the sets of calculations necessary to investigate a given problem. Highly versatile, powerful codes exist to serve the practitioner, but designing useful simulations is a complicated task, involving intricate manipulation of many variables, with many pitfalls for the unwary and the inexperienced. We give an overview of DFT and discuss several of the most important issues that go into designing a meaningful DFT calculation. 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. [Preview Abstract] |
Monday, August 1, 2005 9:15AM - 9:30AM |
B5.00002: Simulations of water in giant planets: discovery of symmetric H-bonding in the superionic phase Nir Goldman, Laurence Fried We detail herein results of \textit{ab initio} Molecular Dynamics simulations of water at conditions close to the isentropes of Neptune and Uranus (temperatures of 1000 - 2000K, and densities of 2.0.-3.0 g/cc). We have calculated the lifetimes and concentrations of molecular and non-molecular species, and ionic conductivity and vibrational spectra. Comparison is made to experiment where possible. At these conditions, we observe the onset of a ``superionic phase'' in which oxygen anions exhibit glassy behavior, and protons diffuse rapidly (10$^{-5}$ - 10$^{-4}$ cm$^{2}$/s) by jumping between oxygen lattice points. We observe two distinct transitions along the superionic phase line. The first consists of a ``dynamically ionized'' phase, wherein H$_{2}$O is the dominant species, but all species lifetimes are exceedingly short-lived such that they are better described as ensembles of transitions states, rather than molecules. At higher densities, we observe a transition to a polymeric phase, in which water has formed partially covalent, symmetric hydrogen bond networks. These results profoundly improve our knowledge of the phase diagram of water. Furthermore they have important implications for the modeling of the interiors of Neptune and Uranus and their corresponding magnetic fields. [Preview Abstract] |
Monday, August 1, 2005 9:30AM - 9:45AM |
B5.00003: Electrical conductivity of shocked water from Density Functional Theory Thomas R. Mattsson, Michael P. Desjarlais We present Density Functional Theory (DFT) calculations of water in a region of phase space of interest in shock experiments. The onset of electrical conductivity in shocked water is determined by ionic conductivity, with the electron contribution dominating at higher pressures. The ionic contribution to the conduction is calculated from proton diffusion (Green-Kubo formula) and the electronic contribution is calculated using the Kubo-Greenwood formula [1]. The calculations are performed with VASP, a plane-wave pseudopotential code. At 2000K and a density of 2.3 g/cc, we find a significant dissociation of water into H, OH, and H3O, not only intermittent formation of OH - H3O pairs as suggested earlier for 2000 K and 1.95 g/cc [2]. The calculated conductivity is compared to experimental data [3]. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Safety Administration under contract DE-AC04-94AL85000. This project was supported by the Sandia LDRD office. [1] M. P. Desjarlais, J. D. Kress, and L. A. Collins; Phys. Rev. B \textbf{66}, 025401 (2002). [2] E. Schwegler, et al. Phys. Rev. Lett. \textbf{87}, 265501 (2001). [3] P.M. Celliers, et. al. Physics of Plasmas \textbf{11}, L41 (2004). [Preview Abstract] |
Monday, August 1, 2005 9:45AM - 10:00AM |
B5.00004: Path Integral Monte Carlo Calculations of Thermodynamic Properties of Dense Hydrogen-Helium Plasma Vladimir Filinov, Pavel Levashov, Michael Bonitz, Vladimir Fortov In this work we present new results of {\it ab initio}\/ calculations of thermodynamic properties of dense hydrogen-helium plasma with helium concentration corresponding to that in the higher layers of the Jovian atmosphere at temperatures from $10^4$~K to $2\cdot 10^5$~K and electron particle densities from $10^{20}$ to $10^{24}$~cm$^{-3}$. The calculations were made by path-integral Monte Carlo method in a cubical cell using periodic boundary conditions. To correctly take into account exchange effects at high values of the degeneracy parameter we used a special correcting procedure. At temperature higher than $5\cdot 10^{4}$~K the calculation results are practically coincides with computations by the equation of state based on the chemical plasma model. However at temperatures $10^4$ and $2\cdot 10^4$~K in the density range from 0.5 to 5~g/cm$^3$ we found a phase transition region positioned in a good agreement with other theories and the experimentally revealed region of the sharp electrical conductivity rise. Along the isotherm $10^4$~K in the density range from 0.01 to 0.2~g/cm$^3$ we found one more region of bad convergence to the equilibrium state. We also present first simulation results for helium plasma in comparison with other models and experimental data. The authors are grateful to the Russian Science support foundation for financial support of the work. [Preview Abstract] |
Monday, August 1, 2005 10:00AM - 10:15AM |
B5.00005: Ab-initio Based Simulations High-Pressure Phases of Nitrogen Federico Zahariev, Anguang Hu, James Hooper, Tom Woo, Fan Zhang The existence of cubic-gauche polymeric form of nitrogen was first predicted theoretically in 1992 and recently confirmed experimentally, further increasing the interest in polynitrogen as a potential high energy density material. We present three new polymeric phases of nitrogen under pressure: 3D chaired six-ring network, 2D boated six-ring layers, 1D chaired six-ring chains. The 3D chaired six-ring network has an enthalpy intermediate between those of black-phosphorus and cubic gauche up to 120GPa. Our simulations provide an insight into the phase transitions of the new metastable phases. [Preview Abstract] |
Monday, August 1, 2005 10:15AM - 10:30AM |
B5.00006: Unified transition path and universal transition state for ZB to RS or WZ to RS high pressure phase transition Maosheng Miao, Walter R.L. Lambrecht We show that the previously proposed transition paths for high pressure phase transitions for semiconductor from zinc blende (ZB) to rocksalt (RS) and from wurtzite (WZ) to rocksalt can be unified and can be extended to transitions from various tetrahedrally bonded polytypes to rocksalt [1]. Our first principle pseudopotential calculations with density functional and constrained relaxation methods on SiC showed that the ZB to RS transition has the lowest transition barrier. Our calculations on ZB to RS transition path for other semiconductors including II-VI, III-V and group IV semiconductors, show that the position and the geometry of the transition state, the state that correspond to the transition barrier, are universal and do not depend on the chemical components of the system [2]. We also extended the Landau phase transition model to a ZB to RS transition by using a cosine function of the atom displacement as order parameter. The model shows that the position of the transition state does not depend on the coupling between the atom displacement and the strains of the lattice, which is the key point that the transition state is independent of the chemical components. [1] M. S. Miao and Walter R. L. Lambrecht, Phys. Rev. B \textbf{68}, 092103 (2003). [2] M. S. Miao and Walter R. L. Lambrecht, Phys. Rev. Lett., accepted [Preview Abstract] |
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