A molecular dynamics study of plasticiser migration in nitrocellulose binders

The migration of the energetic plasticisers 1-nitramino-2,3-dinitroxypropane; 2,4-dinitroethylbenzene; and 2,4,6-trinitroethylbenzene in two nitrocellulose binder mixtures has been investigated by the calculation of diffusion coefficients and activation energies of diffusion from molecular dynamics simulations. The study included parameterisation of force fields for nitrocellulose; 1-nitramino-2,3-dinitroxypropane; the stabilizer ethyl centralite; and the overall nitrocellulose binder mixtures. Simulated densities obtained were in very good agreement with experimental densities. The diffusion coefficients compare favourably with experimental values available for similar systems, when differences such as the proportions of plasticisers are taken into consideration. Examination of the plasticiser diffusion rates suggests that 1-nitramino-2,3-dinitroxypropane migrates more slowly from a nitrocellulose binder than 2,4-dinitroethylbenzene for the nitrocellulose and plasticiser proportions used in this study. Understanding plasticiser migration is essential for the long-term storage of energetic material formulations without significant changes occurring in their properties or compositions

Charge Distributions of Nitro Groups Within Organic Explosive Crystals: Effects on Sensitivity and Modeling

The charge distribution of NO2 groups within the crystalline polymorphs of energetic materials strongly affects their explosive properties. We use the recently introduced basis-space iterated stockholder atom partitioning of high-quality charge distributions to examine the approximations that can be made in modeling polymorphs and their physical properties, using 1,3,5-trinitroperhydro-1,3,5-triazine, trinitrotoluene, 1-3-5-trinitrobenzene, and hexanitrobenzene as exemplars. The NO_{2} charge distribution is strongly affected by the neighboring atoms, the rest of the molecules, and also significantly by the NO2 torsion angle within the possible variations found in observed crystal structures. Thus, the proposed correlations between the molecular electrostatic properties, such as trigger-bond potential or maxima in the electrostatic potential, and impact sensitivity will be affected by the changes in conformation that occur on crystallization. We establish the relationship between the NO_{2} torsion angle and the likelihood of occurrence in observed crystal structures, the conformational energy, and the charge and dipole magnitude on each atom, and how this varies with the neighboring groups. We examine the effect of analytically rotating the atomic multipole moments to model changes in torsion angle and establish that this is a viable approach for crystal structures but is not accurate enough to model the relative lattice energies. This establishes the basis of transferability of the NO2 charge distribution for realistic nonempirical model intermolecular potentials for simulating energetic materials

The ONETEP linear-scaling density functional theory program

We present an overview of the onetep program for linear-scaling density functional theory (DFT) calculations with large basis set (plane-wave) accuracy on parallel computers. The DFT energy is computed from the density matrix, which is constructed from spatially localized orbitals we call Non-orthogonal Generalized Wannier Functions (NGWFs), expressed in terms of periodic sinc (psinc) functions. During the calculation, both the density matrix and the NGWFs are optimized with localization constraints. By taking advantage of localization, onetep is able to perform calculations including thousands of atoms with computational effort, which scales linearly with the number or atoms. The code has a large and diverse range of capabilities, explored in this paper, including different boundary conditions, various exchange–correlation functionals (with and without exact exchange), finite electronic temperature methods for metallic systems, methods for strongly correlated systems, molecular dynamics, vibrational calculations, time-dependent DFT, electronic transport, core loss spectroscopy, implicit solvation, quantum mechanical (QM)/molecular mechanical and QM-in-QM embedding, density of states calculations, distributed multipole analysis, and methods for partitioning charges and interactions between fragments. Calculations with onetep provide unique insights into large and complex systems that require an accurate atomic-level description, ranging from biomolecular to chemical, to materials, and to physical problems, as we show with a small selection of illustrative examples. onetep has always aimed to be at the cutting edge of method and software developments, and it serves as a platform for developing new methods of electronic structure simulation. We therefore conclude by describing some of the challenges and directions for its future developments and applications

The ANANKE Relative-Energy-Gradient (REG) Method to Automate IQA Analysis over Configurational Change

The large volumes of information that arise from telecommunications and cyberspace systems can be represented by massive digraphs. The size of these graphs are so huge that they are unable to be processed by current technologies. The graphs require new and innovative methods of processing and visualizing. Graph surfaces of hierarchical graph slices have been suggested as a way of representing massive digraphs. In this chapter an approach is presented which involves encoding Lipschitz functions into monotone k-logic functions using symmetric chain decompositions (SeD). This approach proposes to address some of the issues concerning huge graphs by providing memory minimization techniques that can be applied to storing graph surfaces