Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session O5: First Principles and Molecular Dynamics VI |
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Chair: Christian Negre, Los Alamos National Laboratory Room: Regency Ballroom B |
Wednesday, July 12, 2017 9:15AM - 9:30AM |
O5.00001: Shock Waves and Defects in Energetic Materials, a Match Made in MD Heaven. Mitchell Wood, David Kittell, Cole Yarrington, Aidan Thompson Shock wave interactions with defects, such as pores, are known to play a key role in the chemical initiation of energetic materials. In this talk the shock response of Hexanitrostilbene (HNS) is studied through large scale reactive molecular dynamics (RMD) simulations. These RMD simulations provide a unique opportunity to elucidate mechanisms of viscoplastic pore collapse which are often neglected in larger scale hydrodynamic models. A discussion of the macroscopic effects of this viscoplastic material response, such as its role in hot spot formation and eventual initiation, will be provided. Through this work we have been able to map a transition from purely viscoplastic to fluid-like pore collapse that is a function of shock strength, pore size and material strength. In addition, these findings are important reference data for the validation of future multi-scale modeling efforts of the shock response of heterogeneous materials. Examples of how these RMD results are translated into mesoscale models will also be addressed. [Preview Abstract] |
Wednesday, July 12, 2017 9:30AM - 9:45AM |
O5.00002: Ab Initio Reactive Monte Carlo Jeffery Leiding, Joshua Coe Equations of state of HE product mixtures under extreme conditions are of obvious interest to the shock physics community. There is also considerable overlap in thermodynamic and chemical space of HE product mixtures and planetary interiors. Chemical equilibrium EOSs provided by thermochemical models are underconstrained due to lack of experimental data, particularly on chemical composition. First-principles data fill this much needed gap. The standard first-principles simulation technique for bulk materials is ab initio molecular dynamics (AIMD). While AIMD is often reliable, I will show that chemical reactions, which are critical to the EOSs of HE product mixtures, can be rare events in AIMD. This causes AIMD to exhibit hysteresis along thermodynamic paths. To solve this problem, we have developed the first ab initio reactive Monte Carlo algorithm (AIRxMC), which samples chemical reactions directly. Standard MC sampling is too inefficient for use with ab initio calculations. I will discuss the techniques used to make the algorithm practical, and compare the results of AIRxMC to those of AIMD for warm dense ammonia, using DFT. [Preview Abstract] |
Wednesday, July 12, 2017 9:45AM - 10:00AM |
O5.00003: Stability of LLM 172 under high pressure. Iskander G. Batyrev, Jennifer A. Ciezak-Jenkins, Gustav M. Borstad LLM-172 or 3, 4-bis (4-nitro-1,2,5- oxadiazol-3-yl)-1,2,5-oxadiazole has been studied experimentally and computationally modeled at high-pressure. Minimum enthalpy structures were relaxed using norm-conserving pseudo potentials which provided a high level of convergence for the final computational structures. The calculated P-V curve fits reasonably well to the experimental X-ray diffraction data. The best fit for calculated values (using Vinet, Birch-Murnaghan, and Tait equations of state) was obtained with bulk modulus K$_{\mathrm{0}}=$17.70 GPa. No phase transitions or deviations from the P2$_{\mathrm{1}}$2$_{\mathrm{1}}$2$_{\mathrm{1}}$ (D2-4) space group of the LLM-172 crystal were observed to near 35 GPa, although slight modifications to the molecular geometry were noted in the Raman spectra. Variational density functional perturbation theory was used to obtain calculated Raman spectra; these calculated spectra were then used for comparison with experimental Raman spectra and the identification of the molecular motions associated with the vibrational modes. Based upon the modification of the experimental Raman spectra with pressure, potential decomposition mechanisms are proposed. The band gap, which was calculated with the GGA-PBE approximation and hybrid functional HSE06, shows a gradual decrease with pressure, although it is far from closing at 30 GPa. [Preview Abstract] |
Wednesday, July 12, 2017 10:00AM - 10:15AM |
O5.00004: Quantifying Ab Initio Equation of State Errors for Hydrogen-Helium Mixtures Raymond Clay, Miguel Morales In order to produce predictive models of Jovian planets, an accurate equation of state for hydrogen-helium mixtures is needed over pressure and temperature ranges spanning multiple orders of magnitude. While extensive theoretical work has been done in this area, previous controversies regarding the equation of state of pure hydrogen have demonstrated exceptional sensitivity to approximations commonly employed in ab initio calculations. To this end, we present the results of our quantum Monte Carlo based benchmarking studies for several major classes of density functionals. Additionally, we expand upon our published results by considering the impact that ionic finite size effects and density functional errors translate to errors in the equation of state. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, July 12, 2017 10:15AM - 10:30AM |
O5.00005: Abstract Withdrawn
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Wednesday, July 12, 2017 10:30AM - 10:45AM |
O5.00006: The Chemistry of Shocked High-energy Materials: Connecting Atomistic Simulations to Experiments. Md Mahbubul Islam, Alejandro Strachan A comprehensive atomistic-level understanding of the physics and chemistry of shocked high energy (HE) materials is crucial for designing safe and efficient explosives. Advances in the ultrafast spectroscopy and laser shocks enabled the study of shock-induced chemistry at extreme conditions occurring at picosecond timescales. Despite this progress experiments are not without limitations and do not enable a direct characterization of chemical reactions. At the same time, large-scale reactive molecular dynamics (MD) simulations are capable of providing description of the shocked-induced chemistry but the uncertainties resulting from the use of approximate descriptions of atomistic interactions remain poorly quantified. We use ReaxFF MD simulations to investigate the shock and temperature induced chemical decomposition mechanisms of polyvinyl nitrate, RDX, and nitromethane. The effect of various shock pressures on reaction initiation mechanisms is investigated for all three materials. We performed spectral analysis from atomistic velocities at different shock pressures to enable direct comparison with experiments. The simulations predict volume-increasing reactions at the shock-to-detonation transitions and the shock vs. particle velocity data are in good agreement with available experimental data. The ReaxFF MD simulations validated against experiments enabled prediction of reaction kinetics of shocked materials, and interpretation of experimental spectroscopy data via assignment of the spectral peaks to dictate various reaction pathways at extreme conditions. [Preview Abstract] |
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