Bulletin of the American Physical Society
17th Biennial International Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 56, Number 6
Sunday–Friday, June 26–July 1 2011; Chicago, Illinois
Session D3: First-Principles and Molecular Dynamics Calculations III: Energetic Materials I |
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Chair: Mark Elert, US Naval Academy Room: Renaissance Ballroom AB |
Monday, June 27, 2011 2:00PM - 2:15PM |
D3.00001: Anisotropic Shock Response of Oriented Nitromethane Single Crystals Lan He, Thomas D. Sewell, Donald L. Thompson Detailed anisotropic structural and mechanical responses of crystalline nitromethane subjected to shock loading along different crystallographic orientations have been studied using molecular dynamics (MD) simulations with a nonreactive force field. Single- and multi-particle properties prior to and following shock passage have been evaluated using a geometric binning approach that spatially and temporally resolves the shock-induced thermo-physical and geometric changes in the material. Initial partitioning and redistribution pathways of the energy imparted by the shock wave result in orientation-dependent structural relaxation processes among which are elastic deformation; crystal structure reordering; and plane-specific disordering phenomena, in which certain structural properties undergo changes from ordered to disordered states in some crystallographic planes but not others. [Preview Abstract] |
Monday, June 27, 2011 2:15PM - 2:30PM |
D3.00002: The effect of a simulated volumetric expansion: Calculated vibrational properties and elastic constants of pentaerythritol J. Criswell, W. Perger, W. Slough, L. Valenzano Current \textit{ab-initio} calculations do not include the van der Waal's interactions. These long range forces are important in the binding of many molecular crystals. Using current theory one may include empirical van der Waal's forces to describe the binding of a molecular solid. The results of \textit{ab-initio} calculations are at 0K. But, experiments measuring material properties are often at ambient conditions. The exclusion of thermal effects produces an inherent disparity between measurements and first-principles calculations of physical properties. In this work, the vibrational spectra and second-order elastic constants (SOECs) of pentaerythritol (PE) are found using density functional theory (DFT) with the B3LYP-D* functional. B3LYP-D* is the B3LYP functional with an empirical description of the van der Waal's dispersion force. PE is chosen because it has a small, highly symmetric, unit cell and exhibits anisotropic binding. Also, recent experimental and theoretical vibrational studies show an interesting behavior of the OH-stretch mode for PE. Using DFT, the SOECs and vibrational spectra of PE are calculated at 0K. The volumetric expansion to ambient temperature is simulated by fixing the unit cell to an experimental volume, optimizing the structure, and recalculating properties. Results of elastic and vibrational properties for 0K, simulated ambient temperature, and experiment are compared. [Preview Abstract] |
Monday, June 27, 2011 2:30PM - 2:45PM |
D3.00003: Inelastic deformation in shock loaded pentaerythritol tetranitrate Reilly M. Eason, Thomas D. Sewell The mechanisms of shock-induced deformation in (100)- and (001)-oriented single crystals of pentaerythritol tetranitrate (PETN) were characterized at the atomic scale using classical molecular dynamics simulations with a fully flexible, non-reactive force field. Shock pressures $P_{shock}\sim $8.7 GPa in initially defect-free, thermalized crystals were studied in order to compare the mechanical behavior for these two orientations under conditions for which neglect of chemistry is a reasonable approximation on the time and space scales of the simulations. A two-wave structure was observed for the (100) shock whereas only elastic compression was observed for the (001) case. Spatially- and temporally-resolved responses for the two orientations were characterized using relative nearest-neighbor molecular displacements, orientational order parameters, dihedral angle distributions, and partitioning of kinetic energy between inter- and intramolecular degrees of freedom. The differences in mechanical response for the two orientations are qualitatively consistent with expectations based on the steric hindrance model of initiation anisotropy. [Preview Abstract] |
Monday, June 27, 2011 2:45PM - 3:00PM |
D3.00004: Effect of impurities on optical properties of Pentaerythritol Tetranitrate (PETN) Roman Tsyshevskiy, Maija Kuklja Optical properties of an PETN molecule and a perfect crystal were studied to provide an interpretation to experimental data concerning to explosive decomposition of PETN caused by Nd:YAG laser irradiation (at 1064nm) (Aluker et all, J Phys Chem, 2011). We established that the HOMO-LUMO gap in a PETN molecule calculated using Gaussian09 program falls into the range of 5.7 to 6.8 eV, and the energy of the lowest singlet-triplet vertical transition requires 3.6-4.3 eV. The band gap of a perfect PETN calculated with VASP code is 4.2 eV. The obtained results show that the optical absorption of an ideal PETN requires the much higher energy than observed in experiment (1.17 eV). This discrepancy rules out the band transitions and suggests that defects induce a new low intensity optical absorption band in PETN. We simulate electronic excitations of possible impurities and compare them to optical properties of both ideal PETN crystals and real samples. Based on the obtained data, we propose a model for the laser initiation of PETN. [Preview Abstract] |
Monday, June 27, 2011 3:00PM - 3:15PM |
D3.00005: Molecular dynamics study of normal mode relaxation and infrared spectra in pentaerythritol tetranitrate Andrey Pereverzev, Thomas D. Sewell Vibrational properties of crystalline pentaerythritol tetranitrate (PETN) were studied using two different approaches based on classical molecular dynamics simulations. In the first approach relaxation of THz-active modes at 300 K and atmospheric pressure was studied by adding fixed amounts of kinetic energy to individual THz-active normal modes of the crystal and monitoring the time dependence of energy transfer from those selectively-excited modes. The THz absorption spectrum was constructed using linewidths obtained directly from the relaxation times of the excited modes for the case of low excitation energy. Dynamics of redistribution of the initial excitation energy among the other normal modes was also studied. In the second approach THz-region infrared spectra were obtained from the Fourier transform of the dipole-dipole autocorrelation function. Predictions of the two approaches are in reasonable agreement with each other and with experiment. Pressure effects and sensitivity to details of the force field were studied using both approaches. [Preview Abstract] |
Monday, June 27, 2011 3:15PM - 3:30PM |
D3.00006: The challenge of predicting physical properties of energetic molecular crystals by first principles calculations Loredana Valenzano, William Slough, Warren Perger The challenge to accurately predict the physical properties of energetic materials is accepted by presenting an ab-initio study on the mechanical and vibrational features of PETN and TATB. While first principles approaches describe properties of materials occurring at 0K, experimental results are mainly provided at 300K. This difference leads to discrepancies in the desired agreement of calculated and measured properties. Also, the congenital limitation of DFT in dealing with long-range dispersion interactions, makes the challenge even more intriguing. Our results are obtained through a full 3D approach that takes into account the van der Waals interaction between the molecules assembled in the solid using the B3LYP-D* functional. Results are compared with experiments showing exceptional agreement and validating the importance of including volume expansion effects in the description of molecular crystals at temperatures above 0K. To the authors' knowledge, the results presented for the elastic constants of TATB are the first ab-initio data for this energetic material. Also, the possibility of using MOFs as a precursor to the detection of explosives is reported. [Preview Abstract] |
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