We present the results of a quantum molecular dynamics simulation of the chemistry of HMX, a high performance explosive, at a density of 1.9 g/cm3 and temperature of 3500 K, conditions roughly similar to the Chapman−Jouget detonation state. The molecular forces are determined using the self-consistent-charge density-functional-based tight-binding method. Following the dynamics for a time scale of up to 55 ps allows the construction of effective rate laws for typical products such as H2O, N2, CO2, and CO. We estimate reaction rates for these products of 0.48, 0.08, 0.05, and 0.11 ps-1, respectively. We also find reasonable agreement for the concentrations of dominant species with those obtained from thermodynamic calculations, despite the vastly different theoretical underpinning of these methodologies.

Decomposition of HMX at extreme conditions: A molecular dynamics simulation

Status and open problems in modeling of as-implanted damage in silicon

Elastic and inelastic processes in the interaction of 1–10 eV ions with solids: ion transport through surface layers

Non-crystalline silicon networks were constructed by atomic deposition and by rapid quenching. The tightbinding molecular dynamics (TBMD) method which had been developed for studying the growth of amorphous carbon structures (Kohary, K., Kugler, S. (2001) “Growth of amorphous carbon: low-energy molecular dynamics (MD) simulation of atomic bombardment,” Phys. Rev. B 63 193404), was applied. During the relaxations the temperature versus time functions display stretched exponential forms. The most important structural properties were analyzed. The final networks contain significant numbers of triangles and squares.

/pdf/growth-of-amorphous-carbon-low-energy-molecular-dynamics-48ciqsoazu.pdf

Growth of amorphous carbon: Low-energy molecular dynamics simulation of atomic bombardment

We have extended our experimentally constrained molecular relaxation technique [P. Biswas et al., Phys. Rev. B 71, 54204 (2005)] to hydrogenated amorphous silicon: a 540-atom model with 7.4% hydrogen and a 611-atom model with 22% hydrogen were constructed. Starting from a random configuration, using physically relevant constraints, ab initio interactions, and the experimental static structure factor, we construct realistic models of hydrogenated amorphous silicon. Our models confirm the presence of a high-frequency localized band in the vibrational density of states due to Si-H vibration that has been observed in recent vibrational transient grating measurements on plasma enhanced chemical vapor deposited films of hydrogenated amorphous silicon.

/pdf/experimentally-constrained-molecular-relaxation-the-case-of-4y7uq8zh5g.pdf

Experimentally constrained molecular relaxation: The case of hydrogenated amorphous silicon

We describe $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ using a tight-binding Hamiltonian which is based on s, p, and d orbitals for Si and s orbitals for H. The sample is prepared by quenching a Si (6.25%) H mixture from the gas phase at zero pressure. The condensation phase transition is clearly observed at roughly 2500 K. The resulting $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ specimen has a density of 2.35 ${\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$; Si atoms are mainly fourfold coordinated, with around 7% fivefold coordinated atoms, while H atoms exhibit a tendency for clustering. As a specific example for a dynamic event, we study the structural rearrangements occurring after breaking a Si-H bond.

Tight-binding molecular-dynamics study of a-Si:H: Preparation, structure, and dynamics

Institut fu ¨r Physik, Theoretische Physik III, Technische Universitat, D-09107 Chemnitz, Germany~Received 21 February 1996!The growth of a reconstructed ~100!-silicon crystal during the deposition of 4 ML of 2-eV silicon atoms isstudied with special emphasis on the structure of the grown material. Two different molecular-dynamicssimulation methods are employed: A classical scheme using the Stillinger-Weber potential, and a density-functional-based tight-binding scheme devised by Frauenheimet al. @Phys. Rev. B 52, 11 492 ~1995!#.Wemonitor the density, pair correlation, bond and dihedral angle distribution, and ring statistics. Clear differencesin the structure of the material grown by the two different simulation schemes are observed. They can be tracedback to a too large stiffness of the classical potential, which leaves the grown material in a disordered but nottruly amorphous state. @S0163-1829~96!03324-3#I. INTRODUCTION

Comparison of classical and tight-binding molecular dynamics for silicon growth.

We have studied the transmission of low-energy (10 eV) ${\mathrm{F}}^{+}$, ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$, and ${\mathrm{F}}_{2}^{\mathrm{\ensuremath{-}}}$ ions through ultrathin Kr and Xe films. The ions are produced by electron-stimulated desorption from a ${\mathrm{PF}}_{3}$-covered Ru(0001) surface at 25 K, and their yields and angular distributions are measured with a digital, angle-resolving ion detector. The rare-gas films are condensed onto the ${\mathrm{PF}}_{3}$ layer, and the yield and angular distribution of the ions are measured as a function of the rare-gas film thickness. We find that ${\mathrm{F}}^{+}$ ions are attenuated nearly completely by \ensuremath{\sim}1 ML of Kr or Xe, and attribute this to both one-electron charge transfer and elastic scattering. Surprisingly, we find an increase in ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ yield for the first rare-gas layer with respect to the clean surface value, which is accompanied by a dramatic change in the ion angular distribution. The ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ yield decreases to zero around 2.5 ML Kr or Xe; we explain the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ attenuation and the change in the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ angular distributions in an elastic-scattering model. The increase in yield is attributed to a reduction in the neutralization probability of ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ with the surface in the presence of the rare-gas layer. \textcopyright{} 1996 The American Physical Society.

Inelastic and elastic processes in the transmission of F + , F - , and F - 2 from PF 3 /Ru(0001) through thin rare-gas films

Tight-binding simulation of liquid and amorphous Si at zero pressure

By use of molecular dynamics we simulate a recent experiment by Sack et al. in which oxygen ions are desorbed by electron impact from a ${\mathrm{WO}}_{\mathit{x}}$ surface, and their transmission through rare-gas films of a few monolayer thickness has been measured. When using only elastic scattering cross sections between ${\mathrm{O}}^{+}$ and rare-gas atoms in the simulation, we find fair quantitative agreement with the measured transmission yield, and energy, and angular distributions, for Xe and Kr films. The large number of transmitted oxygen ions can be traced back to the small ${\mathrm{O}}^{+}$ radius. The discrepancy between measured and simulated transmission behavior through Ar films is attributed to either electronically inelastic losses or structural effects in these films. The im pact of this work on the depth of origin of sputtered particles is discussed.

Transmission of low-energy oxygen ions through ultrathin rare-gas films: Molecular-dynamics simulation.

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