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 H3: First-Principles and Molecular Dynamics Calculations V: Energetic Materials II |
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Chair: Ramon Ravelo, University of Texas at El Paso Room: Renaissance Ballroom AB |
Tuesday, June 28, 2011 9:15AM - 9:30AM |
H3.00001: The terahertz infrared spectrum of cyclotrimethylenetrinitramine: targeting anharmonic modes for the fingerprinting and detection of RDX William Slough, Loredana Valenzano, Warren Perger Recent approaches to the modeling of molecular solids have provided for a dramatic improvement in the prediction of zero Kelvin behavior for some properties of interest. Most notably, the vibrational spectrum for these systems can now be calculated robustly via ab initio methods employing density functional theory. This improvement, however, leads to a quandary: the accurate physical modeling of these systems at zero Kelvin in many cases will not provide values and even physical behavior matching experimental values under ambient conditions. We examine this quandary in detail by considering zero Kelvin calculations using the B3LYP-D* functional of the terahertz infrared spectrum of the energetic material cyclotrimethylenetrinitramine (RDX). Most importantly we show what knowing the deviation from the simple harmonic approximation of a given mode at zero Kelvin says about the anharmonicity of the mode near ambient volumes. Finally, we discuss the practical implications for using ab initio calculations to create ?finger-prints? for the detection of explosives such as RDX. [Preview Abstract] |
Tuesday, June 28, 2011 9:30AM - 9:45AM |
H3.00002: Mechanism change in hot dense liquid nitromethane decomposition: ReaxFF molecular dynamics simulations Naomi Rom, Sergey Zybin, Adri van Duin, William Goddard, Yehuda Zeiri, Gil Katz, Ronnie Kosloff The decomposition mechanism of hot liquid nitromethane (NM) at various compressions and temperatures was studied using reactive force field (ReaxFF) molecular dynamics simulations. A competition between two initial thermal decomposition schemes is observed, depending on compression. At low densities unimolecular C-N bond cleavage is the dominant route, whereas when approaching Chapman-Jouget detonation conditions the dominant mechanism switches to the formation of CH$_{3}$NO fragment. The change in decomposition mechanism of hot liquid NM leads to different kinetic and energetic behavior and products distribution. [Preview Abstract] |
Tuesday, June 28, 2011 9:45AM - 10:00AM |
H3.00003: Gamma phase RDX: vibrational features providing insight into the alpha-gamma phase transition Ken Flurchick, Warren Perger, William Slough, Loredana Valenzano We present a full 3D periodic density functional theory study of the infrared spectra of gamma phase RDX. The B3LYP-D* functional as adjusted for molecular solids from Grimme's semi-empirical approach for molecules is used to better describe the van der Waals interactions in this system. Specifically, the low terahertz portion of the spectrum is explored in detail for modes that change substantially in behavior with respect to an alpha phase that is near the transition pressure for the system. These key modes provide possible clues into the nature of the alpha-gamma phase transformation. We also present infrared reflectance spectra and results for the refractive index, with the intent to guide experiments in achieving a more complete understanding of the vibrational features of this energetic molecular crystal. [Preview Abstract] |
Tuesday, June 28, 2011 10:00AM - 10:15AM |
H3.00004: Comparative analysis of decomposition reactions in gaseous and crystalline $\beta $-HMX Onise Sharia, Maija Kuklja Most quantum chemical studies focus on determining reaction paths and reaction barriers. We will illustrate that knowing only those parameters is insufficient for finding the dominant mechanism. One needs to calculate both the activation barriers and the reaction rates. We present a density functional theory based modeling of several possible detonation initiation reactions in HMX, including its gas phase, a perfect crystal, and a crystal containing vacancies, voids or internal surfaces. We show that the N-NO$_{2}$ homolysis is the most favorable decomposition reaction in the gas phase. In the crystalline phase, this reaction has a higher activation barrier and becomes much slower due to the densely packed structure of HMX. As a result, two other reactions, the HONO elimination and NONO rearrangement, would compete with the N-NO$_{2}$ homolysis in an ideal crystal. Practical samples however contain a lot of imperfections hence we also studied an effect of voids on the chemical decomposition. We established that a large space in the vicinity of voids facilitates the N-NO$_{2}$ break, and, similarly to the gas phase, the N-NO$_{2}$ reaction proceeds with the highest rate. The conclusions and revealed trends help to provide a consistent interpretation to experimental data. [Preview Abstract] |
Tuesday, June 28, 2011 10:15AM - 10:30AM |
H3.00005: Calculation of the vibrational spectra of RDX as a function of pressure using the Grimme DFT potential Warren Perger, K.M. Flurchick, Wil Slough, Loredana Valenzano The density-functional theory (DFT) potential by Grimme has been proposed for describing long-range dispersion corrections. This potential has been implemented into the CRYSTAL09 program and used to calculate the vibrational spectra in RDX at equilibrium and as a function of pressure. The intensities, Born charge tensor, and high-frequency dielectric constant are reported and compared with prior theory and experiment where possible. [Preview Abstract] |
Tuesday, June 28, 2011 10:30AM - 10:45AM |
H3.00006: ABSTRACT WITHDRAWN |
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