W91INF-05-1-0265 REPORT NUMBER REPORT ~'u~mER 48101-EG-MUR The views, opinions andlor findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other documentation. Approved for public release; federal purpose rights The response of the energetic molecular crystal cyclotrimethylene trinitramine (RDX) to the propagation of planar shock waves nonnal to (100) has been studied using large-scale molecular dynamics simulations that employ an accurate and transferable nonreactive potential. The propagation of the shock waves was simulated using nonequilibrium molecular dynamics. Shear bands were nucleated during shocks with a particle velocity of 1.0 km s-I and corresponding Rankine-Hugoniot shock pressun of9.7 GPa. These defects propagate into the compressed material at 45° to (100) in the (010) zone. The shear bands e\'oh'e slowly compared to the time scales routinely accessible to nonequilibrium molecular dynamics toward a liquidlike state as a result of viscous heating. A recently developed shock-front absorbing boundary condition [AV. Bolesta et al. , Phys. Rev. B 76, 224108 (2007)] was applied to the simulation cells at the moment of maximum compression to sustain the shock-compressed state. Molecular dynamics simulations were then employed to study the temIXlral and structural evolution of the shock-induced shear bands toward a steady-fluctuating state. Owing to the intense, viscous flow-driven heati ng within the shear bands. these defects can be considered to be homogeneously nucleated hot spots.

Anomalous hardening under shock compression in (021)-oriented cyclotrimethylene trinitramine single crystals Atomic-scale analysis of defect dynamics and strain relaxation mechanisms in biaxially strained ultrathin films of face-centered cubic metalsThe propagation of shock waves normal to ͑111͒ in the energetic molecular crystal cyclotrimethylene trinitramine ͑RDX͒ has been studied using large-scale molecular dynamics simulations. Partial dislocation loops with Burgers vector 0.16͓010͔ are nucleated homogeneously on ͑001͒ at Rankine-Hugoniot shock pressures greater than 1.3 GPa. Calculations of the ͓010͔ cross-section of the ͑001͒ generalized stacking fault energy surface as a function of applied pressure along ͓001͔ reveals that the stacking fault enclosed by the partial dislocation loops is rendered metastable by a stress-induced change in molecular conformation. Furthermore, large-scale molecular dynamics simulations performed on quasi-two-dimensional ͑111͒-oriented single crystals show a two-wave elastic-plastic response with a "galloping" plastic wave. We propose that the onset of homogeneous dislocation nucleation accounts for the abrupt change in the elastic-plastic response of macroscopic ͑111͒-oriented RDX single crystals observed in recent experiments by giving rise to an anomalous plastic hardening.

Born-Oppenheimer molecular dynamics simulations with long-term conservation of the total energy and a computational cost that scales linearly with system size have been obtained simultaneously. Linear scaling with a low pre-factor is achieved using density matrix purification with sparse matrix algebra and a numerical threshold on matrix elements. The extended Lagrangian Born-Oppenheimer molecular dynamics formalism [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] yields microcanonical trajectories with the approximate forces obtained from the linear scaling method that exhibit no systematic drift over hundreds of picoseconds and which are indistinguishable from trajectories computed using exact forces.

Extended Lagrangian Born-Oppenheimer molecular dynamics based on Kohn-Sham density functional theory is generalized in the limit of vanishing self-consistent field optimization prior to the force evaluations. The equations of motion are derived directly from the extended Lagrangian under the condition of an adiabatic separation between the nuclear and the electronic degrees of freedom. We show how this separation is automatically fulfilled and system independent. The generalized equations of motion require only one diagonalization per time step and are applicable to a broader range of materials with improved accuracy and stability compared to previous formulations.

A simple model of atomic interactions in noble metals based explicitly on electronic structure,

New parametrizations for semiempirical density functional tight binding (DFTB) theory have been developed by the numerical optimization of adjustable parameters to minimize errors in the atomization energy and interatomic forces with respect to ab initio calculated data. Initial guesses for the radial dependences of the Slater-Koster bond integrals and overlap integrals were obtained from minimum basis density functional theory calculations. The radial dependences of the pair potentials and the bond and overlap integrals were represented by simple analytic functions. The adjustable parameters in these functions were optimized by simulated annealing and steepest descent algorithms to minimize the value of an objective function that quantifies the error between the DFTB model and ab initio calculated data. The accuracy and transferability of the resulting DFTB models for the C, H, N, and O system were assessed by comparing the predicted atomization energies and equilibrium molecular geometries of small molecules that were not included in the training data from DFTB to ab initio data. The DFTB models provide accurate predictions of the properties of hydrocarbons and more complex molecules containing C, H, N, and O.

The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity to mesoscale structure, and has been a topic of extensive study for over 50 years.

The interaction and diffusion of lithium atoms in a (5,5) carbon nanotube is studied using density-functional theory. The Li-nanotube interaction perpendicular to the tube axis for a single Li inside and outside the tube is calculated and compared with the Li-graphene interaction obtained using the same technique. Both interactions are similar in the repulsive region but exhibit differences in their attractive part. Nevertheless, they can be described using a common parametrization. The Li-Li interaction is calculated as a function of their separation inside the tube. This interaction is similar to a screened repulsive Coulomb potential at small separations. However, at larger separations, the Li-Li interaction does not vanish and shows residual oscillations. This repulsive long-ranged interaction favors concerted diffusion of many Li atoms compared to the independent diffusion of individual Li inside the tube. The interaction and diffusion of lithium atoms in a ͑5,5͒ carbon nanotube is studied using density-functional theory. The Li-nanotube interaction perpendicular to the tube axis for a single Li inside and outside the tube is calculated and compared with the Li-graphene interaction obtained using the same technique. Both interactions are similar in the repulsive region but exhibit differences in their attractive part. Nevertheless, they can be described using a common parametrization. The Li-Li interaction is calculated as a function of their separation inside the tube. This interaction is similar to a screened repulsive Coulomb potential at small separations. However, at larger separations, the Li-Li interaction does not vanish and shows residual oscillations. This repulsive long-ranged interaction favors concerted diffusion of many Li atoms compared to the independent diffusion of individual Li inside the tube. Disciplines Engineering | Materials Science and Engineering Comments