Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S23: Simulations of Matter at Extreme Conditions 
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Sponsoring Units: DCOMP GSCCM DMP Chair: Keith Runge, University of Florida Room: 202B 
Thursday, March 5, 2015 8:00AM  8:12AM 
S23.00001: The importance of the finitetemperature exchangecorrelation for warm dense matter studies V.V. Karasiev, S.B. Trickey Matter at extremely elevated temperature (thousands to millions Kelvin) under a wide range of pressures usually is treated by ab initio molecular dynamics driven by freeenergy DFT. Whether in the KohnSham or orbitalfree forms, implementation requires a reliable exchangecorrelation (XC) free energy approximation. Finitetemperature HartreeFock calculations [1] suggest strongly that the explicit Tdependence of X is important. The recently developed first rung XC freeenergy functional, the finiteT local density approximation (LDA) [2], captures that explicit Tdependence for the homogeneous electron gas. We report study of the impact of explicit Tdependence in the LDA on the properties of matter in the warm dense regime and conclude that there is a need to develop a Tdependent and density gradientdependent XC functional. Next, we analyze the finiteT gradient expansion for X and C, extract from it the appropriate reduced density gradients for X and C with explicit Tdependence, introduce the nextrung GGA XC freeenergy functionals, and discuss their behavior and properties. \\[4pt] [1] V.V. Karasiev, T. Sjostrom, and S.B. Trickey, Phys. Rev. E 86, 056704 (2012).\\[0pt] [2] V.V. Karasiev, T. Sjostrom, J. Dufty, and S.B. Trickey, Phys. Rev. Lett. 112, 076403 (2014). [Preview Abstract] 
Thursday, March 5, 2015 8:12AM  8:24AM 
S23.00002: Theoretical Investigation of Differing Ion and Electron Temperatures in Hydrogen Keith Runge, Valentin Karasiev, Pierre Deymier Andrew Ng [1] has reported experimental results for silicon and gold in warm, dense matter conditions that have been modeled by use of distinct temperatures for the ions and electrons. Here we investigate the implications of such differing ion and electron temperatures for hydrogen using pathintegral molecular dynamics (PIMD) for protons which interact with electrons descibed by orbitalfree density functional theory (OFDFT). Temperatures ranging from room temperature to many kilokelvin are considered for the protons, while the electron temperatures are fixed in the kilokelvin range. Recent advances [2] in OFDFT not only allow for faster generation of first principles forces but also include the effects of temperature on the electron density. Comparisons are made with classical molecular dynamics to allow us to quantify the quantum nuclear effects captured by PIMD.\\[4pt] [1] A. Ng, Int. J. Quantum Chem., 112, 150 (2012).\\[0pt] [2] V. V. Karasiev, D. Chakraborty, O. A. Shukruto, and S. B. Trickey, Phys. Rev. B 88, 161108(R) (2013). [Preview Abstract] 
Thursday, March 5, 2015 8:24AM  8:36AM 
S23.00003: Highthroughput calculations of pressureinduced phase transitions in tungsten nitride Michael Mehl, Daniel Finkenstadt, Gus Hart, Stefano Curtarolo We have previously\footnote{M. Mehl {\em et al.}, arXiv:1403.2762 [condmat.mtrlsci] (2014)} used highthroughput electronic structure calculations\footnote{S. Curtarolo {\em et al.}, http://materials.duke.edu/aflow.html} to determine the ground state structures of the tungstennitride system as a function of composition. In doing this, we found many structures with are close to the WN convex hull and apparently metastable. The question then arises if any of these structures can be stabilized under pressure. To test this, we have determined the ground state hull as a function of pressure up to 50~GPa. We find that the structures on the hull change with pressure. We discuss some of the more interesting structures, and show how the choice of density functional changes our predictions. [Preview Abstract] 
Thursday, March 5, 2015 8:36AM  8:48AM 
S23.00004: Selfdiffusion and viscosity for warm dense systems by orbitalfree density functional theory Travis Sjostrom, Jerome Daligault Evaluation of transport properties requires significantly longer molecular dynamics simulations than, for example, equation of state calculations. The standard approach at lower temperatures is to use KohnSham (orbital dependent) density functional theory to find the quantum electron density at every molecular dynamics step. However, the KohnSham approach becomes computationally prohibited at higher temperatures for equation of state, let alone for transport properties. Our recent orbitalfree approach [Phys. Rev. Lett . 113, 155006 ] has shown excellent agreement with KohnSham method at lower temperatures and extends to very high temperatures . Here we evaluate selfdiffusion and viscosity from low to high temperatures and compare with KohnSham methods where applicable as well as with recent approaches of kinetic theory. [Preview Abstract] 
Thursday, March 5, 2015 8:48AM  9:00AM 
S23.00005: A New ForceMatched Reactive Force Field for Bulk Water Under Extreme Thermodynamic Conditions Laurence Fried, Lucas Koziol A manybody classical force field is presented for water under dissociative thermodynamic conditions. The force field is optimized by forcematching to ab initio molecular dynamics (AIMD) simulations calculated with Density Functional Theory (DFT). The force field contains shortranged central terms, manybody overcoordination terms, and longrange Ewald electrostatics. It is optimized and tested on water at density 1.5 g/mL and 2000 K, which is approximately 10\% dissociated according to DFT. Molecular dynamics simulations closely reproduce DFT radial distribution functions, as well as the distribution of ${\rm H_2O}$ and dissociation products. The calculated atomic selfdiffusion constants appear about 50\% lower than in DFT, although precise comparison is impossible due to the short timescale accessible to AIMD (about 20 ps). The force field is also compared to ReaxFF using the CHO parameter set of Chenowith et al. ReaxFF structural and dynamical properties are in overall fair agreement with DFT, although ReaxFF water is not dissociative at these conditions. [Preview Abstract] 
Thursday, March 5, 2015 9:00AM  9:12AM 
S23.00006: N Dependence of the Equilibrium Free Energy from the Canonical Ensemble Debajit Chakraborty, James Dufty Freeenergy density functional theory conventionally is formulated in the Grand Canonical ensemble but implemented computationally in the Canonical ensemble. To investigate the effects of this disjuncture, the equilibrium free energy per particle for a uniform noninteracting gas is calculated from the Canonical ensemble for given particle number $N$ and fixed volume. The same calculation is performed from the Grand Canonical ensemble for the corresponding average $\overline{N}$ and same volume. In dimensionless forms the latter depends only on the reduced temperature, $t=T/T_{F}$. In contrast, of course, the Canonical result depends on both $t$ and $N$. The results are compared for $1$$<$$N$$<$$100$. Next, the same calculations are performed for a nonuniform gas generated by an external ion using a regularized Coulomb potential. The number dependence is explored for positions near and far from the ion. The implications for freeenergy density functional theory are discussed. [Preview Abstract] 
Thursday, March 5, 2015 9:12AM  9:24AM 
S23.00007: Pressurestabilized lithium cesides with cesium anions beyond the 1 state Jorge Botana, MaoSheng Miao Main group elements usually assume a typical oxidation state while forming compounds with other species that depends on the occupation of the outermost orbital. Group I elements are usually in the $+$1 state in inorganic materials. Our recent work on CsF compounds reveals that pressure may make the inner shell 5p electrons of Cs reactive, causing Cs to expand beyond the $+$1 oxidation state. In our study, we have found that pressure can cause large electron transfer from light alkali metals such as Li to Cs, causing Cs to become anionic with a formal charge much beyond 1. Li and Cs only form alloys at ambient conditions, but by studying the thermodynamic stability of the intermetallic compounds Li$_{n}$Cs$_{m}$ (n$=$15, m$=$1; n$=$1,m$=$14), we have found that some Li$_{n}$Cs (n$=$1,3,4,5) compounds become stable under pressures higher than 100 GPa. Once formed, these compounds exhibit interesting structural features, including capped cuboids and dimerized icosahedra. Finally, we have also found superconductivity in metastable LiCs and that the unusual anionic state of Cs has a strong effect on the transition temperature. [Preview Abstract] 
Thursday, March 5, 2015 9:24AM  9:36AM 
S23.00008: Bypassing the malfunction junction in warm dense matter simulations Attila Cangi, Aurora PribramJones Simulation of warm dense matter requires computational methods that capture both quantum and classical behavior efficiently under hightemperature and highdensity conditions. The stateoftheart approach to model electrons and ions under those conditions is density functional theory molecular dynamics, but this method's computational cost skyrockets as temperatures and densities increase. We propose finitetemperature potential functional theory as an inprincipleexact alternative that suffers no such drawback[1]. In analogy to the zerotemperature theory developed previously[2,3], we derive an orbitalfree free energy approximation through a couplingconstant formalism. Our density approximation and its associated free energy approximation demonstrate the method's accuracy and efficiency.\\[4pt] [1] A. Cangi and A. PribramJones, arXiv:{\bf 1411.1532} (2014).\\[0pt] [2] A. Cangi, D. Lee, P. Elliott, K. Burke, and E.K.U. Gross, PRL {\bf 106}, 236404 (2011).\\[0pt] [3] A. Cangi, E.K.U. Gross, and K. Burke, PRA {\bf 88}, 062505 (2013). [Preview Abstract] 
Thursday, March 5, 2015 9:36AM  9:48AM 
S23.00009: Interaction of Short Pulse Laser Irradiation with Metal Targets under Condition of Spatial Confinement Maxim Shugaev, Eaman Karim, ChengYu Shih, Chengping Wu, Leonid Zhigilei While the general mechanisms of laser interactions with metals in vacuum have been broadly investigated experimentally, theoretically and computationally, the effect of strong spatial confinement by solid or liquid overlayers on the laser induced processes remains largely unexplored. In this work, the results of largescale atomistic simulations of short pulse laser irradiation of metal targets in liquid environment and in the presence of a transparent silica glass overlayer are used to reveal the effect of the spatial confinement on the material response to the fast laser energy deposition. The ability of the overlayer to suppress generation of unloading tensile wave, prevent photomechanical spallation and explosive decomposition of the surface region of the target, and facilitate the formation of a high pressure and temperature supercritical state near the interface is revealed in the simulations. The results of the simulations clarify the mechanisms responsible for structural and morphological changes in the interfacial region, formation of voids and crystal defects, and generation of nanoparticles. [Preview Abstract] 

S23.00010: ABSTRACT WITHDRAWN 
Thursday, March 5, 2015 10:00AM  10:12AM 
S23.00011: Modeling Instability Growth in Inertial Confinement Fusion Simulations Jeremy Melvin, Verinder Rana, Hyunkyung Lim, Baolian Cheng, James Glimm, David Sharp, Doug Wilson We numerically investigate the mixing behavior of RayleighTaylor (RT) and RichtmyerMeshkov (RM) instabilities during the implosion of Inertial Confinement Fusion (ICF) capsules. We emphasize the importance of accurate modeling of the RM growth as an initial seed to the RT growth. Using models to predict the mixing parameters (Prandtl, Schmidt, Reynolds), we provide estimates of the projected growth of these instabilities and discuss their impact on ignition conditions in the hot core. In addition, we discuss the impact of adding front tracking capabilities to ICF codes and their modeling of instability growth. [Preview Abstract] 
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