Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session M2: First Principles and Molecular Dynamics Calculations III |
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Chair: Tim Germann, Los Alamos National Laboratory Room: Fairmont Orchid Hotel Amphitheater |
Wednesday, June 27, 2007 10:30AM - 10:45AM |
M2.00001: Molecular Dynamics Studies of Thermal Induced Chemistry in TATB Jason Quenneville, Timothy Germann A reactive force field (ReaxFF$^{2})$ is used with molecular dynamics to probe the chemistry induced by intense heating (accelerated `cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. Using small, 800-atom, simulations, we will show the reaction rate as a function of temperature and density as well as the time evolution of reaction products. A larger simulation (with 14,000 atoms) will illustrate the effect of system size on both carbon clustering and reaction rate. Insight into the mechanisms of product formation will be given, as well as the chemical structure (graphitic or diamond-like) of the carbon clusters obtained. $^{2}$ A. C. T. Van Duin, et al, \textit{J. Phys. Chem. A}, \textbf{1005}, 9396 (2001). [Preview Abstract] |
Wednesday, June 27, 2007 10:45AM - 11:00AM |
M2.00002: Initial steps of condensed-phase decomposition of TATB from reactive molecular dynamics Hyungjun Kim, Sergey Zybin, Adri van Duin, William Goddard The initial steps of condensed-phase decomposition of TATB high explosive have been investigated by molecular dynamics method using ReaxFF reactive force field parameterized from the first-principles calculations. We study the dependence of primary and secondary reaction kinetics on the initial temperature and density both in pure crystal and in presence of various defects such as voids and inclusions. We found that at lower temperature the primary decomposition steps mainly involve intramolecular hydrogen transfer followed by the formation of water molecules while at higher temperatures the homolytic cleavage of N-NO2 bond can appear, providing a temperature-dependent effect on the decomposition pathway. Besides, we also observe the formation of carbonaceous clusters during thermal and shock induced decomposition in TATB that initially capture large amount of the oxygen and nitrogen atoms delaying secondary reactions of formation of small molecules and further transformation of the cluster into carbon soot. We analyze the primary and secondary reaction mechanisms as well as the evolution of decomposition products in TATB crystals under various thermal and shock conditions. [Preview Abstract] |
Wednesday, June 27, 2007 11:00AM - 11:30AM |
M2.00003: Atomistic Studies of Energetic Materials Under Shock Wave Loading Invited Speaker: We have recently undertaken non-equilibrium molecular dynamics studies to aid our understanding of dynamical processes in the high explosives HMX and RDX, in particular the inelastic, anisotropic response of crystals of those materials subjected to quasi-static and shock loading. The overarching goal of this work is to provide information that can serve as a foundation in basic science for the formulation of improved mesoscale constitutive models for the constituent materials in selected energetic formulations. The medium-term scientific challenge to this larger objective is to carefully identify, characterize, and quantify the dominant mechanisms of localization and dissipation in such materials, under a variety of prescribed quasi-static and dynamic loading scenarios that lead to inelastic deformation of the crystals. The focus of the present talk will be the unreactive shock response of structurally perfect, but properly thermalized, HMX and RDX crystals; and shock localization in defective RDX crystals. Effort will be made to present information of interest to the shock physics, materials science, and chemical dynamics/spectroscopy communities. [Preview Abstract] |
Wednesday, June 27, 2007 11:30AM - 11:45AM |
M2.00004: Shear-strain Sensitivity of Energetic Crystals and the Origin of Hot-spots Maija Kuklja, Sergey Rashkeev Simulation of shear-induced chemical reactions of decomposition of crystalline FOX-7 and TATB is performed by means of Density Functional Theory and First Principles Molecular Dynamics. It is shown that the shear-strain deformation plays a crucial role in defining the sensitivity of explosive crystals to initiation and strongly depends on the shape of crystalline layers constituting the materials. Energetic barriers for FOX-7 decomposition are found to decrease due to shear while those for TATB are not affected by this deformation. We discuss possible mechanisms of chemistry in hot spots, associated with the local shear-strain deformation. This work made possible to provide specific recommendations for synthesis of insensitive energetic materials. [Preview Abstract] |
Wednesday, June 27, 2007 11:45AM - 12:00PM |
M2.00005: Molecular dynamics simulation of the shock-induced wurtzite-to-rocksalt transition in CdSe and CdS Aidan Thompson, Marcus Knudson The shock-induced wurtzite-to-rocksalt structural transformation is studied using large-scale molecular dynamics simulation. The primary goal is to understand the atomistic mechanisms underlying the interesting transformation kinetics observed in the case of cadmium sulfide [M. D. Knudson and Y. M. Gupta, J. Appl. Phys, v. 91, p. 9561, 2002]. Since the mechanical and structural properties of cadmium selenide are similar to those of cadmium sulfide, as a first step multi-million atom shock propagation simulations have been carried out in CdSe using the Rabani force field, which has been shown to correctly describe the important bulk phases of CdSe, as well as the wurtzite-to-rocksalt transformation pressure. As a next step a force field for CdS will be developed to enable comparison of similar shock propagation simulations with experiments. [Preview Abstract] |
Wednesday, June 27, 2007 12:00PM - 12:15PM |
M2.00006: Molecular Dynamics Simulation of TATB-like Explosive Filipp Sapozhnikov, Vladimir Dremov, Ilya Derbenev, Alexey Karavaev, Laurent Soulard A modification of REBO potential has been proposed for the molecular dynamics simulation of a TATB-like condensed explosive whose molecule initially consists of four different atoms. TATB-like means bulk properties of initial state and parameters at CJ point similar to those of real TATB. Parameters of the potential are subdivided into two groups that are responsible for CJ parameters and reaction zone width. The possibility of formation of intermediate detonation products allows variation of reaction zone characteristics without changing CJ parameters. Provided are a number of test MD calculations on the thermodynamic properties of both the original explosive and detonation products, parameters at CJ point, reactions rates and reaction zone width as dependent upon the potential parameters as well as the evaluation of critical diameter. Mechanism of the detonation initiation proper to heterogeneous explosives has been investigated. [Preview Abstract] |
Wednesday, June 27, 2007 12:15PM - 12:30PM |
M2.00007: Elastic and thermodynamic properties of post-perovskite MgSiO$_{3}$ from first-principles calculations Liu Zi-Jiang The elastic and thermodynamic properties of post-perovskite MgSiO3 polymorph are investigated at high pressures and temperatures using the plane wave pseudopotential method within the local density approximation. This phase may be the most abundant mineral in the D" region. It is found that the post-perovskite phase has similar bulk modulus and larger shear modulus than perovskite at relevant pressures. The athermal elastic constant tensor of post-perovskite MgSiO3 are calculated as a function of pressure up to 200 GPa. The calculated results are in excellent agreement with other predictions over the pressure regime studied. The thermodynamic properties of post-perovskite MgSiO3 polymorph are predicted using the quasi-harmonic Debye model; the heat capacity and thermal expansion coefficient accord with the other calculations at high pressures and temperatures. [Preview Abstract] |
Wednesday, June 27, 2007 12:30PM - 12:45PM |
M2.00008: Predicting noncovalent interactions with nonlocal density functional theory Joe Hooper, Frank Zerilli, Nic Romero, Betsy Rice We report calculations using a new, nonlocal density functional which explicitly treats long-range van der Waals correlation in a nonempirical way. This method is designed to model complex electron interactions such as dispersion, an area where traditional DFT methods often perform poorly. The functional performs quite well for certain types of noncovalent bonding, producing energies within a few percent of high-level CCSD(T) and MP2 methods at a fraction of the calculation time. The nonlocal correlation scales appropriately under compression, reducing to accurate GGA results at intramolecular length scales. Intermolecular distances are generally overpredicted compared to coupled-cluster results; this is largely an artifact of the GGA exchange component, and can be improved by incorporating exact exchange. This new method appears to be quite promising for treating complex organic molecular crystals at a range of pressures. [Preview Abstract] |
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