Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K2: Materials in Extremes VFocus

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Sponsoring Units: DCOMP DMP SHOCK Chair: Stephane Mazavet, Observatoire de Paris Room: 261 
Wednesday, March 15, 2017 8:00AM  8:36AM 
K2.00001: Transport properties of an asymmetric mixtures in the dense plasma regime Invited Speaker: Christopher Ticknor We study how concentration changes ionic transport properties along isobars for mixtures of hydrogen and various higher charge elements, Li, C, Al, Cu, and Ag, representative of turbulent layers relevant to inertial confinement fusion and astrophysics. Hydrogen will typically be fully ionized while other elements will be only partially ionized but can have a large effective charge. This will lead to very different physical conditions for the H and the other elements. Large first principles orbital free molecular dynamics simulations are performed, and the resulting transport properties are analyzed. Comparisons are made with transport theory in the kinetic and coupled regimes. The addition of a small amount of heavy element in a light material has a dramatic effect on viscosity and diffusion of the mixture. [Preview Abstract] 
Wednesday, March 15, 2017 8:36AM  8:48AM 
K2.00002: Firstprinciples calculation of K absorption edge of warm dense aluminum Shen Zhang, Wei Kang, Ping Zhang, X. T. He Shifts of K absorption edge of warm dense aluminum with respect to the change of thermodynamic conditions are calculated using firstprinciples methods\footnote{S. Zhang, S. Zhao, W. Kang, P. Zhang, and X. T. He, Phys. Rev. B \textbf{93}, 115114 (2016)}, which shows good agreement with the experimental measurements\footnote{A. BenuzziMounaix, F. Dorchies, V. Recoules, F. Festa, O. Peyrusse, A. Levy, A. Ravasio, T. Hall, M. Koenig, N. Amadou, E. Brambrink, and S. Mazevet, Phys. Rev. Lett. \textbf{107}, 165006 (2011)}. Improvement is achieved with a careful consideration of the effect of core electrons. The shift of K edge is revealed more sensitive to the change of temperature than to that of density in the warm dense region, which suggests K edge shift a good temperature indicator. The flattening point of the K edge shift at higher temperature is attributed to the thermal depletion of the Mband electrons. [Preview Abstract] 
Wednesday, March 15, 2017 8:48AM  9:00AM 
K2.00003: Classical and quantum simulations of warm dense carbon Heather Whitley, David Sanchez, Sebastien Hamel, Alfredo Correa, Lorin Benedict We have applied classical and DFTbased molecular dynamics (MD) simulations to study the equation of state of carbon in the warm dense matter regime ($\rho =$ 3.7 g/cc, 0.86 eV \textless T \textless 100 eV). We utilize two different classical interatomic potentials: 1. LCBOP, designed to simulate solid phases of C, and 2. linearly screened Coulomb (Yukawa) potentials. We observe that LCBOP overpredicts the pair correlations in liquidC in this regime when compared to the DFTMD results. Conversely, the Yukawa model seems to produce the correct qualitative features in the static ionic pair distributions at the highestT, but does not capture the correct correlations at lower T. However, both interaction potentials predict that the decay in the ionic contribution of the specific heat as T approaches infinity is much slower than that predicted by a model based on DFTMD. These differences in the MDderived equations of state in warm dense regimes could have important consequences when using classical interionic forces such as these in largescale MD simulations aimed at studying processes of relevance to inertial confinement fusion. This study points to a need for better interatomic potentials to describe warm dense matter. Prepared by LLNL under Contract DEAC5207NA27344. [Preview Abstract] 
Wednesday, March 15, 2017 9:00AM  9:12AM 
K2.00004: Firstprinciples study of equation of state and shock compression of warm dense sodium and other lowZ materials Shuai Zhang, Kevin Driver, François Soubiran, Burkhard Militzer At high pressure, sodium exhibits exotic properties that deviate from a simple metal. Knowledge of its equation of state (EOS) under simultaneous high pressures and temperatures are still scarce. Following our recent work on hot, dense silicon (Militzer and Driver, PRL 115 (2015) 176403), here we investigate various nodal surfaces in path integral Monte Carlo (PIMC) and reconfirm that localized, HartreeFock orbitals yield accurate pressures and internal energies for hot, dense sodium. We combine PIMC and density functional theory molecular dynamics (DFTMD) in order to derive a coherent, firstprinciples EOS for sodium over a wide range of densities (1.9311.60 g/cm\textasciicircum 3) and temperatures (10\textasciicircum 41.29x10\textasciicircum 9 K). The EOS allows us to study sodium under shock compression. Fully including the electron shell and excitation information, our firstprinciples EOS predicts the shock Hugoniot curve to have two compression maxima: one below and one above 10\textasciicircum 6 K corresponding to the ionization of L and K shell electrons. In this regard our predictions differ from empirical EOS database (SESAME and LEOS), which show only a single peak in compression. Results for other lowZ materials will also be presented. We predict sodium to have an unusually high compression ratio along the principal Hugoniot curve. [Preview Abstract] 
Wednesday, March 15, 2017 9:12AM  9:24AM 
K2.00005: KuboGreenwood Electric Conductivity Tensor:\\ Essentials and Opensource Implementation Lazaro Calderin, Valentin V Karasiev, James Dufty, Samuel B Trickey We survey the essentials for calculation of the complex electric conductivity tensor based on the KuboGreenwood (KG) formalism, then discuss our new implementation of a KG postprocessing tool for Quantum Espresso. The motivating physical problem is computational characterization of warm dense matter. Solutions to both formal and technical problems implicit in the use of Projector Augmented Wave datasets and a plane wave basis will be discussed, especially in the context of simulations of materials in extreme conditions. A full implementation in an opensource Fortran 90 code and illustrative results from it will be presented. [Preview Abstract] 
Wednesday, March 15, 2017 9:24AM  9:36AM 
K2.00006: Tunable noninteracting freeenergy functionals: development and applications to lowdensity aluminum Samuel Trickey, Valentin Karasiev We introduce the concept of tunable orbitalfree noninteracting freeenergy density functionals and present a generalized gradient approximation (GGA) with a subset of parameters defined from constraints and a few free parameters. Those free parameters are tuned to reproduce reference KohnSham (KS) staticlattice pressures for Al at T=8 kK for bulk densities between 0.6 and 2 g/cm$^3$. The tuned functional then is used in OF molecular dynamics (MD) simulations [1] for Al with densities between 0.1 and 2 g/cm$^3$ and T between 6 and 50 kK to calculate the equation of state and generate configurations for electrical conductivity calculations [2]. The tunable functional produces accurate results. Computationally it is very effective especially at elevated temperature. KohnShiam calculations for such low densities are affordable only up to T=10 kK, while other OF approximations, including twopoint functionals, fail badly in that regime. 1. V.V. Karasiev, T. Sjostrom, and S.B. Trickey, Comput. Phys. Commun. 185, 3240 (2014). 2. V.V. Karasiev, L. Calder{\'i}n, and S.B. Trickey, Phys. Rev. E 93, 063207 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 9:36AM  9:48AM 
K2.00007: Learning a force field for the martensitic phase transformation in Zr Hongxiang Zong, Ghanshyam Pilania, Rampi Ramprasad, Turab Lookman Atomic simulations provide an effective means to understand the underlying physics of martensitic transformations under extreme conditions. However, this is still a challenge for certain phase transforming metals due to the lack of an accurate classical force field. Quantum molecular dynamics (QMD) simulations are accurate but expensive. During the course of QMD simulations, similar configurations are constantly visited and revisited. Machine Learning can effectively learn from past visits and, therefore, eliminate such redundancies. In this talk, we will discuss the development of a hybrid MLQMD method in which ondemand, onthefly quantum mechanical (QM) calculations are performed to accelerate calculations of interatomic forces at much lower computational costs. Using Zirconium as a model system — for which accurate atomisctic potentials are currently unvailable — we will demonstrate the feasibility and effectiveness of our approach. Specifically, the computed structural phase transformation behavior within the MLQMD approach will be compared with available experimental results. Furthermore, results on phonons, stacking fault energies, and activation barriers for the homogeneous martensitic transformation in Zr will be presented. [Preview Abstract] 
Wednesday, March 15, 2017 9:48AM  10:00AM 
K2.00008: Ground state of lithium: evidence from the de Haasvan Alphen effect analysis Sabri Elatresh, Weizhao Cai, Neil Ashcroft, Roald Hoffmann, Shanti Deemyad, Stanimir Bonev The lithium Fermi surface in the bcc, fcc, hcp, and 9R structures is computed at zero pressure and temperature using first principles theory. It is shown that measurements of the Fermi surface based on the de Haasvan Alphen effect can be used as a diagnostic method to investigate the low temperature phase diagram of lithium. The theoretical results are presented in conjunction with experimental data, which could allow us to rule out some of the phases as the ground state structure of lithium at zero pressure. [Preview Abstract] 
Wednesday, March 15, 2017 10:00AM  10:12AM 
K2.00009: Hotspot criticality in amorphous and crystalline RDX  role of thermal conduction and initial free energy Alejandro Strachan, Michael Sakano, Brenden Hamilton We use molecular dynamics (MD) simulations with the reactive force field ReaxFF to characterize the criticality of nanoscale hot spots and crystalline and amorphous RDX. This is motivated by the recent finding that hotspots formed from the dynamical collapse of a pore are significantly more reactive than thermally created ones with identical size and thermodynamic conditions. The increased reactivity of the dynamical hotspot can be attributed to mechanochemistry, reactions away from local equilibrium or the fact that the material around the dynamical hotspot is disordered and, thus, possesses higher free energy and lower thermal conductivity. We studied hotspots of various sizes and temperatures and follow their evolution to determine conditions under which they transition to a deflagration wave. Our preliminary results indicate that hotspots in amorphous RDX are more reactive and, for a give size, transition to a deflagration wave at lower temperatures. We discuss the possible origins of this increased reactivity in terms of thermal transport that tends to quench the hotspot and the free energy of the initial states. [Preview Abstract] 
(Author Not Attending)

K2.00010: Molecular Dynamics Simulations of Shear Induced Transformations in Nitromethane James Larentzos, Brad Steele Recent experiments demonstrate that NM undergoes explosive chemical initiation under compressive shear stress. The atomistic dynamics of the shear response of singlecrystalline and bicrystalline nitromethane (NM) are simulated using molecular dynamics simulations under high pressure conditions to aid in interpreting these experiments. The atomic interactions are described using a recently reoptimized ReaxFFlg potential trained specifically for NM under pressure. The simulations demonstrate that the NM crystal transforms into a disordered state upon sufficient application of shear stress; its maximum value, shear angle, and atomicscale dynamics being highly dependent on crystallographic orientation of the applied shear. Shear simulations in bicrystalline NM show more complex behavior resulting in the appearance of the disordered state at the grain boundary. [Preview Abstract] 
Wednesday, March 15, 2017 10:24AM  10:36AM 
K2.00011: Nonequilibrium dynamics due to moving deflagration front at RDX/HTPB interface Santanu Chaudhuri, Kaushik Joshi, Naida Lacevic Reactive dissipative particle dynamics (DPDRX), a promising tool in characterizing the sensitivity and performance of heterogeneous solid propellants like polymer bonded explosives (PSXs), requires further testing for nonequilibrium dynamics. It is important to understand detailed atomistic chemistry for developing coarse grain reactive models needed for the DPDRX. In order to obtain insights into combustion chemistry of RDX/HTPB binder, we used reactive molecular dynamics (RMD) to obtain energy uppumping and reaction mechanisms at RDX/HTPB interface when exposed to a selfsustaining deflagration front. Hot spots are ignited near and away from the heterogeneous interface using the thermal pulse. The results show that the hot spot near interface significantly delays the transition from ignition to deflagration. We will present the mechanical response and the combustion chemistry of HTPB when the propagating deflagration front hits the polymer binder. We will discuss our efforts to incorporate this RMD based chemistry into the DPDRX which will enable us to perform such nonequilibrium dynamics simulations on largelength scale with microstructural heterogeneities. [Preview Abstract] 
Wednesday, March 15, 2017 10:36AM  10:48AM 
K2.00012: Phase transformation pathways of Ln$_{\mathrm{2}}$O$_{\mathrm{3}}$ irradiated by ultrafast laser Dylan Rittman, Jonathan Solomon, Curtis Chen, Cameron Tracy, Steven Yalisove, Mark Asta, Wendy Mao, Rodney Ewing Ultrafast laser irradiation induces highly nonequilibrium conditions in materials through intense electronic excitation over very short timescales. Here, we show that ultrafast laser irradiation drives an irreversible cubictomonoclinic phase transformation in Ln$_{\mathrm{2}}$O$_{\mathrm{3}}$ (Ln $=$ ErLu). A combination of grazing incidence Xray diffraction and transmission electron microscopy is used to characterize the amount and depthdependence of the phase transformation. Results indicate thatalthough all materials experience the same transformationit is achieved through different damage mechanisms (pressure vs. thermal), and the short timescales associated with damage provides nonequilibrium routes of material modification. Ab initio molecular dynamics are used to isolate the effects of electronic excitations, and results are shown to be consistent with the trend in radiation resistance observed experimentally. Overall, this study provides a path to gain insight into the relationship between a material's equilibrium phase diagram and its behavior under highly nonequilibrium conditions. [Preview Abstract] 
Wednesday, March 15, 2017 10:48AM  11:00AM 
K2.00013: Low Parallax High Brightness Imaging at High Laser Repetition Rates Kevin McNesby, Barrie Homan A new laser illuminator has been used to image detonations of chemical explosions at rates exceeding 200 kHz. The new laser illuminator is a hybrid fiber/Nd:YAG laser and is part of a Low Parallax High Brightness Imaging (LPHBI) facility used to study shock wave formation and energy release during testing of chemical explosives up to a net explosive weight of 10 kg. The laser provides pulses of monochromatic radiation (wavelength 532 nanometers) that produces a shadowgraph of an explosive event, captured using a high speed camera (Photron SAz) filtered and timesynchronized to the laser output. Using this technique, shock formation following energy release, shock detachment from explosive products, and shock reflection may be visualized in real time. The new laser replaces a copper vapor laser used at the facility since 2004. The new laser enables increases in temporal resolution up to a factor of 10 (from 50 microseconds between images to 5 microsecond between images). The discussion will include a description of the ways the new system is being used to investigate time to metal participation in energy release for realscale explosive charges. [Preview Abstract] 
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