Ab Initio Study of Compressed 1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane (HMX), Cyclotrimethylenetrinitramine (RDX), 2,4,6,8,10,12-Hexanitrohexaazaisowurzitane (CL-20), 2,4,6-Trinitro-1,3,5-benzenetriamine (TATB), and Pentaerythritol Tetranitrate (PETN)

Abstract

Using the PW91, PBE, and LDA density functional theories (DFT), we have calculated crystal structures for five energetic molecular crystals over a range of experimental pressures. These crystals are 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), cyclotrimethylenetrinitramine (RDX), 2,4,6,8,10,12-hexanitrohexaazaisowurzitane (CL-20), 2,4,6-trinitro-1,3,5-benzenetriamine (TATB), and pentaerythritol tetranitrate (PETN). Both PW91 and PBE generally overestimate volumes relative to experimental values, while LDA underestimates crystal volumes when compared to experiment. However, the inaccuracy diminishes as pressures are increased. In particular, PW91 and PBE volumes approach experimental values at pressures greater than 6−7 GPa. Furthermore the PW91 and PBE volumes appear to converge to the same value as pressures increase, regardless of the size of the planewave basis set. We have also demonstrated that for systems such as these, care should be taken to ensure convergence in DFT calculations. We emphasize that the mistreatment of van der Waals forces by DFT will have unforeseen consequences on all crystal calculations for weakly bound organic molecules, and therefore caution should be employed whenever interpreting results obtained from the current DFT functionals available in solid-state codes.

Experimental and DFT-D Studies of the Molecular Organic Energetic Material RDX

“…21 A subsequent study demonstrated that an increase in pressure diminishes the importance of the dispersion interactions relative to the increasing contribution of the repulsive interactions. 22 As the external pressure applied to the simulation cell increased, the inaccuracies of the predicted intermolecular distances and lattice parameters relative to experimental data decreased, to the extent that good agreement with experiment was produced for pressures greater than 6-7 GPa. 21,22 Shimojo et al 24 have shown that the dispersion correction implemented by Grimme 25 accounts for the dispersion interactions accurately for the -RDX crystal in the high-pressure regime (0 -15 GPa), while incurring little additional computational overhead.…”

“…22 As the external pressure applied to the simulation cell increased, the inaccuracies of the predicted intermolecular distances and lattice parameters relative to experimental data decreased, to the extent that good agreement with experiment was produced for pressures greater than 6-7 GPa. 21,22 Shimojo et al 24 have shown that the dispersion correction implemented by Grimme 25 accounts for the dispersion interactions accurately for the -RDX crystal in the high-pressure regime (0 -15 GPa), while incurring little additional computational overhead. In addition, the authors determined that the non-empirical van der Waals density-functional (vdW-DF) method also provides an accurate description of the vdW interactions, but requires orders-of-magnitude more computational resource.…”

Analysis of thermomechanical response of polycrystalline HMX under impact loading through mesoscale simulations

“…[1][2][3][4][5] Heating due to each of these mechanisms depends on the histories of the states of local stress and rate of deformation in the material. Existing literature examines the behavior of energetic crystals and polymer-bonded explosives (PBXs) in two general forms: as single crystals with anisotropic material properties, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and as heterogeneous collections of energetic grains with isotropic material properties. [23][24][25][26][27] The single crystal simulations have been carried out using both discrete methods (e.g., molecular dynamics (MD) and density functional theory (DFT)) [6][7][8][9][10][11][12][13][14][15][16] and continuum scale methods.…”

“…Existing literature examines the behavior of energetic crystals and polymer-bonded explosives (PBXs) in two general forms: as single crystals with anisotropic material properties, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] and as heterogeneous collections of energetic grains with isotropic material properties. [23][24][25][26][27] The single crystal simulations have been carried out using both discrete methods (e.g., molecular dynamics (MD) and density functional theory (DFT)) [6][7][8][9][10][11][12][13][14][15][16] and continuum scale methods. [17][18][19][20][21] Analyses utilizing discrete methods have yielded understanding of many of the thermo-mechanical properties of the polymorphs of HMX including crystal structure and lattice parameters, 11 isotherms, 10 elastic constants, 7 viscosity, 6 thermal conductivity, and behavior in compression.…”

First-principles thermodynamics of energetic materials

“…Initial efforts focused on calculating equations of state (EOSs) and thermal properties using pure density functional theory (DFT) methods; however, DFT with generalized gradient approximation (GGA) greatly over-predicted equilibrium volumes by an average of 10% compared to experiment due to inadequate description of dispersive van der Waals (vdW) interactions [3,4]. More recently, attempts to economically incorporate vdW into DFT (DFT+vdW) led to various treatments using pair-wise C 6 r -6 potentials that are simply added to DFT at large distances [5,6].…”