Chapter 7 Structure and density predictions for energetic materials

Abstract

This chapter focuses on predictions for energetic materials. Because of the importance of crystal density in the performance of energetic materials, the initial efforts were directed to this area in general and to volume additivity techniques in particular. The chapter focuses on the procedures that have evolved over a period of about 20 years. The chapter discusses the development of molecular packing (MOLPAK). The first step in developing a method for predicting crystal structures is to characterize the coordination spheres found in known structures. This is limited to the compounds of moderate molecular weight containing only C, H, N, O, and F in the triclinic, monoclinic, and Z = 4 orthorhombic space groups. An initial survey of coordination sphere information used crystal structure information for 136 carbons to fluorine-containing materials and is later expanded to 267. The composition of each coordination sphere (the molecules are Identity or Initial, Plane, Axis and Center) is characteristic of the space group and the three-dimensional relationships among the molecules fall into a limited number of patterns and sub-patterns. The repulsion energy between two (or more) molecules is computed as the sum of all pair-wise atom-to-atom interactions. The threshold energy has been derived from known structures by searching for values that reproduced the true unit cell volume with the central molecule in the orientation of the known structure. The chapter provides the examples of coordination sphere building procedures, examples of MOLPAK + WMIN predictions and there is also structure prediction. In the chapter, the procedures for crystal structure (and density) prediction have been detailed. Difluoroamino compounds are of interest because of their potential high density and energy and properties as solid propellant oxidizers. The crystal structure “shows an interesting, unpredicted feature: channels, with a 3-fold axis of symmetry surrounded by HNFX molecules.”