Session O4: TM Molecular Dynamics I

Atomic-Scale Theoretical Studies of Fundamental Properties and Processes in CHNO Plastic-Bonded Explosive Constituent Materials under Static and Dynamic Compression

The results of recent theoretical atomic-scale studies of CHNO plastic-bonded explosive constituent materials will be presented, emphasizing the effects of static and dynamic compression on structure, vibrational spectroscopy, energy redistribution, and dynamic deformation processes. Among the chemical compounds to be discussed are pentaerythritol tetranitrate (PETN), hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX), nitromethane, and hydroxyl-terminated polybutadiene (HTPB). Specific topics to be discussed include pressure-dependent terahertz IR absorption spectra in crystalline PETN and RDX, microscopic material flow characteristics and energy localization during and after pore collapse in shocked (100)-oriented RDX, establishment of local thermodynamic temperature and the approach to thermal equilibrium in shocked (100)-oriented nitromethane, and structural changes and relaxation phenomena that occur in shocked amorphous \textit{cis}-HTPB. In the case of shocked HTPB, comparisons will be made between results obtained using fully-atomic and coarse-grained (united atom) molecular dynamics force field models. Rather than attempting to discuss any given topic in extended detail, 3-4 vignettes will be presented that highlight outstanding scientific questions and the predictive methods and tools we are developing to answer them.   

Fast Quantum Molecular Dynamics Simulations of Simple Organic Liquids under Shock Compression

The responses of liquid formic acid, acrylonitrile, and nitromethane to shock compression have been studied using quantum-based molecular dynamics simulations with the self-consistent tight-binding code LATTE. Microcanonical Born-Oppenheimer trajectories with precise energy conservation were computed without relying on an iterative self-consistent field optimization of the electronic degrees of freedom at each time step via the Fast Quantum Mechanical Molecular Dynamics formalism [A. M. N. Niklasson and M. J. Cawkwell, Phys. Rev. B, \textbf{86}, 174308 (2012)]. The input shock pressures required to initiate chemistry in our simulations agree very well with recent laser- and flyer-plate-driven shock compression experiments. On-the-fly analysis of the electronic structure of the liquids over hundreds of picoseconds after dynamic compression revealed that their reactivity is strongly correlated with the temperature and pressure dependence of their HOMO-LUMO gap.   

Quantum molecular dynamics simulations of the stability and reactivity of aluminum cyclopentadienyl clusters

We report \textit{ab initio} quantum molecular dynamics simulations of the thermal stability and oxidation of aluminum-cyclopentadienyl clusters currently being considered as novel fuels or energetic materials. These clusters contain a small aluminum core surrounded by a single organic ligand layer. The aromatic cyclopentadienyl ligands form a very strong bond with surface Al atoms, giving rise to a stable organometallic cluster which crystallizes into a low-symmetry solid-state material. Our calculated heat of combustion per unit volume of this solid is quite high (60{\%} that of pure aluminum) with reaction kinetics potentially much faster than nanoscale metals. Though this compound can be experimentally produced in small quantities, it is quite volatile. Here we report theoretical studies of the stability and decomposition of these passivated aluminum clusters in the presence of oxygen using Car-Parrinello molecular dynamics. We also consider alternate ligand forms which offer significantly increased steric protection or contain groups (such as fluorine) which may react with the metallic core.   

Reactive Molecular Dynamics Simulation of Hotspot Formation in Shock-Induced Void Collapse of Pentaerythritol Tetranitrate (PETN)

We present results of molecular dynamics simulations of hotspot formation in shock-induced void collapse of pentaerythritol tetranitrate (PETN) with the ReaxFF reactive force field. A supported shockwave is driven through a PETN crystal along the [110] orientation; void size and piston velocity are varied to investigate their effects on hotspot formation and detonation initiation. Formation of hotspots during void collapse is characterized by hotspot extent and maximum temperature. Results show hotspot extent is directly related to void size, but maximum temperature is only slightly affected. On the other hand, both hotspot extent and maximum temperature are strongly dependent upon piston velocity. Hotter and larger hotspots facilitate detonation and subsequent chemical reactions. During void collapse, NO$_{\mathrm{X}}$ molecules are shown to be the dominant ejectile from the upstream void surface. Once the void is filled and the hotspot develops, formation of final products such as N$_{2}$ and H$_{2}$O become more dominant.   

Molecular dynamics simulation of the burning front propagation in PETN

One of the models of detonation development in condensed explosives under shock loading is the concept of ``hot spots.'' According to this model, the reaction initially starts at various defects and inhomogeneities, where energy is localized during shock wave propagation. In such a region the exothermic reaction may start with heat yield sufficient for the ignition of the adjacent layers of matter. If the reaction propagates fast enough, the merging of the burning fronts from several hot spots may lead to detonation. So there is an interest in determining the burning propagation rate from the hot spot in various conditions. In this work we investigate the propagation of plane burning front from initially heated layer in PETN single crystal using molecular dynamics method with reaction force field (ReaxFF). It is shown that the burning rate depends on the direction in crystal. The kinetics of chemical transformations is considered, main reaction paths are determined. The dependence of the burning front propagation rate on the external pressure in the pressure range of normal to 30 GPa is calculated, it is shown that it grows linearly in the considered range from 50 m/s to 320 m/s. The results are compared with the data from experiments and quantum chemical calculations.