Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H13: Focus Session: Simulations of Matter at Extreme Conditions III: Classical MD, Potentials, and Energetic Materials |
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Sponsoring Units: DCOMP GSCCM Chair: Carter White, Naval Research Laboratory Room: Morial Convention Center 204 |
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H13.00001: Theoretical Approach for Developing Accurate Potentials for Molecular Dynamics Simulations: Thermoelastic Response of Aluminum J.M. Winey, A. Kubota, Y.M. Gupta To achieve the correct thermoelastic response of solids in classical simulations, a new approach is presented for developing accurate interatomic potentials. In this approach, the potentials are fitted to values for the atomic volume and the second- and third-order elastic constants at T = 0K by extrapolating the room temperature values, using classical thermo-mechanical relations. This procedure avoids the low- temperature quantum regime, enabling recovery of the correct response in classical simulations to higher temperature. As an example of this approach, an EAM potential was developed for aluminum. Results using this potential provide consistently better agreement with thermoelastic data at higher temperature compared to previous EAM potentials. Our approach is applicable to the development of other types of potentials as well and is amenable to incorporating the results of first principles calculations performed using the classically extrapolated volume for T = 0K. Work supported by DOE. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H13.00002: Petaflop simulations of shock-induced particulate ejection from copper Timothy C. Germann, James E. Hammerberg, Guy Dimonte We present the results of several large-scale, Non-Equilibrium Molecular Dynamics (NEMD) simulations of shock-induced surface instability development. We consider single crystal Cu described by an embedded atom method (EAM) potential and driven by a shock wave along the [111] crystallographic direction, impinging upon a roughened Cu/vacuum or Cu/Ne interface. The initial temperature is 300K and the NEMD simulation cell is a quasi-2D $2.23\mu m \times 5.67 \mu m$ slab geometry, 1.5 nm thick in the (periodic) third dimension. The first third of the sample length ($1.89 \mu m$) is occupied by Cu ($5.3 \times 10^{8}$ atoms), and the remainder either empty vacuum or Ne gas at a pressure of 0.67 MPa ($1.95 \times 10^{8}$ atoms). The Cu/Ne (or Cu/vacuum) interface has an initial perturbation with average amplitude 30 nm and dominant wavelength of $0.74 \mu m$. A shock wave is created by driving the free end of the Cu slab at a fixed particle velocity $u_p =$ 2.0 to 3.5 km/s. Single-mode and multi-mode interfaces were considered using 212,992 CPUs of the LLNL BlueGene/L supercomputer for times approaching 1 ns. At the higher particle velocities, the Cu release state is in the fluid-solid mixed phase. We discuss the evolution of the density and velocity distributions of the ejected mass, the modes of particle breakup, and comparisons to source theories of ejecta formation and Richtmyer-Meshkov instabilities. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H13.00003: Interatomic bond-order potentials for molecular dynamics simulations of materials at extreme conditions Romain Perriot, Mikalai Budzevich, Ivan Oleynik Molecular dynamics (MD) is a powerful research tool for studying materials at extreme conditions. At the heart of MD are the interatomic potentials, whose quality in describing a variety of chemical effects, including bond-breaking and bond-making, plays a decisive role in delivering meaningful results. We have performed extensive MD simulations of shock compression of covalently bonded materials, such as diamond and silicon, and found that REBO interatomic potential for diamond and EDIP potential for Si have substantial deficiencies at large pressures and temperatures in spite of the fact that the near equilibrium properties of both diamond and silicon are well reproduced. We are addressing this outstanding issue by developing analytic bond-order potentials (BOPs) specifically for the simulation of covalently bonded materials at extreme conditions. These BOPs include explicit analytic expressions for both the $\sigma $ and $\pi $ bonds. We will discuss important steps of BOP construction which includes devising a fist-principle database of fundamental materials properties, fitting this database by the orthogonal tight-binding, and devising the analytic BOPs using the direct link between TB and analytic BOPs via the bond orders. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H13.00004: Interatomic Potentials for Large-Scale Simulations of High-Pressure, High-Temperature Phenomena Invited Speaker: The use of large-scale atomistic simulations in the study of high-compression, high strain-rate phenomena has dramatically increased in the last decade. Most of this type of simulations utilize classical empirical or semi-empirical potentials to describe the inter-atomic interactions. The regime of validity of most of these potentials is however often limited to a narrow region of the pressure-temperature phase diagram. In the development of accurate inter-atomic potentials for material simulations at high-pressures or temperatures, a high degree of transferability is desirable without resorting to fitting everywhere in phase space. We will review two popular cluster functional models: the embedded-atom-method (EAM) and the modified embedded-atom-method (MEAM). The embedded atom method provides a very good description of metallic properties at a low computational cost and has become the workhorse of large-scale atomistic simulations of metallic systems. The modified embedded-atom-method is an improvement of EAM which includes the effect of angular bonding. We outline inherent limitations of these models and present a systematic approach to improving their transferability and predictive accuracy at high pressures and/or temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H13.00005: Ultrafast semi-metallic layer formation in detonating nitromethane Evan Reed, M. Riad Manaa, Laurence Fried, Kurt Glaesemann, John Joannopoulos We present the first quantum molecular dynamics simulations behind a detonation front (up to 0.2 ns) of the explosive nitromethane (CH$_3$NO$_2$) represented by the density-functional-based tight-binding method (DFTB). This simulation is enabled by our recently developed multi-scale shock wave molecular dynamics technique (MSST) that opens the door to longer duration simulations by several orders of magnitude. The electronic density of states around the Fermi energy initially increases as metastable material states are produced but then later decreases, perhaps unexpectedly. These changes indicate that the shock front is characterized by an increase in optical thickness and conductivity followed by a reduction around 100 picoseconds behind the front. We find that a significant population of intermediate metastable molecules are charged and charged species play an important role in the density of states evolution. The transient transformation to a semi-metallic state can be understood within the Anderson picture of metallization. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H13.00006: Reactive Molecular Dynamics Studies of Thermal Induced Chemistry in HMX Jason Quenneville, Timothy Germann, Thomas Sewell, Edward Kober Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemistry induced by intense heating (`cook-off') of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers$^{1}$ at CalTech and has shown promise in predicting the chemistry in a variety of systems, including RDX and TATB under either shock compression or intense heat. In the current work, we investigate the effect of initial equilibration temperature (1000 to 1500 K), volumetric compression, crystal polymorph ($\beta $ and $\delta )$, and system size (ranging from 150 to 1200 molecules) on the reaction rate and reaction products. Finally, we will compare these results with those from our previous work on TATB. $^{1}$A. C. T. Van Duin, \textit{et al}, \textit{J. Phys. Chem. A}, \textbf{1005}, 9396 (2001). [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H13.00007: Anisotropic Constitutive Relationships in Energetic Materials: TATB Mikalai M. Budzevich, Aaron Landerville, Mike Conroy, Ivan I. Oleynik, Carter T. White One of the principal thrusts in energetic materials (EM) research is the acquisition of accurate equations of state (EOS) for various important classes of EMs. In the past, both theoretical and experimental studies concentrated on hydrostatic EOS. However, these isotropic EOS still need to be expanded to include anisotropic materials response, including uniaxial compression which are more relevant to shock initiation of detonation. To this end, we performed first-principles density functional calculations of the EOS for TATB, including uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] crystallographic directions. Equilibrium properties, such as lattice parameters and elastic constants, as well as the hydrostatic EOS were calculated and compared with experimental results. Finally, we discuss the possible relationship between shear stresses induced by the uniaxial compression of TATB and the relative shock sensitivities of different crystallographic directions. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H13.00008: First-Principles Reactive Molecular Dynamics of Chemistry in Detonating Energetic Materials Aaron Landerville, Ivan I. Oleynik, Mortko A. Kozhushner, Carter T. White We investigated the initial chemistry of shock compressed energetic materials that results from inter-molecular collisions behind the shock wave front by performing first-principles MD simulations of bimolecular collisions for PETN and RDX with different crystallographic orientations and velocities. For each orientation, we determined the threshold collision velocity for reaction, the reaction timescales, and the products of decomposition. We find that the calculated threshold velocities lie within the range of typical particle flow velocities in detonating materials. Owing to the extremely short reaction timescales ($\sim $ 10$^{-13 }$s), these initial chemical events are largely driven by the direct collision dynamics, instead of temperature. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H13.00009: Reactive MD simulations of anisotropic response of PETN under high-rate shear deformation Xu Peng, Sergey Zybin, Aidan P. Thompson, William A. Goddard Several experiments have indicated that the shock sensitivity of single crystal energetic materials can depend on the crystallographic direction. We develop a compress-and-shear modeling approach to study the mechanisms of anisotropic shock sensitivity using the ReaxFF reactive molecular dynamics. ReaxFF is a first-principles based force field capable to reproduce the quantum chemical energies of the reactants, products, intermediates and transition states with functional forms suitable for large-scale molecular dynamics simulations of chemical reactions under extreme conditions. In this presentation we will discuss the results of high-rate shear simulations of uniaxially compressed PETN. We found noticeable differences in the physical and chemical responses of PETN for different combinations of the slip system and compression direction. The simulation results agree well with the experimental shock-initiation sensitivity data and Dick's steric hindrance theory. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H13.00010: First-Principles Constitutive Relationships in PETN and HMX under Hydrostatic and Uniaxial Compressions Sergey V. Zybin, Michael W. Conroy, Ivan I. Oleynik, Carter T. White The physical mechanisms leading to shock-induced detonation at the atomic level are ultimately related to energetic materials response to uniaxial compression at the shock front. Due to the intrinsic anisotropy of the constitutive relationships, a description of the compressed state should be extended beyond hydrostatic equations of state that are frequently used for analysis of precursor states of EMs. In this presentation, we will discuss the results of first-principles density functional theory calculations of both hydrostatic and uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] directions applied to the energetic materials PETN-I and $\beta $-HMX. A comparison will be made of available experimental data with calculated physical properties such as unit cell geometry, isothermal equations of state, and elastic constants. The presentation will focus on the anisotropic nature of the constitutive relationships in molecular crystals under uniaxial compression. The behavior of the shear stress projections on available slip systems upon uniaxial strain and their possible relationship to experimental shock-initiation sensitivity data will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H13.00011: Reactive molecular dynamics simulations of shocked PETN Joanne Budzien, Aidan P. Thompson, Sergey V. Zybin We have performed molecular dynamics simulations of PETN crystals subjected to shock along the [100] direction. Using the reactive forcefield, ReaxFF, and the molecular dynamics code, GRASP, allows us to track the chemical reactions that occur as both a function of time and position. By simulating larger systems, we can observe the formation of both primary and secondary products to make comparisons with experiments. Composition profiles of these products will be shown along with profiles of stress, temperature, and potential energy. [Preview Abstract] |
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