The two constituents HMX and CL-20 produce different effects on the crystal structure of their cocrystal in different directions.

Density functional theory with dispersion-correction (DFT-D) was employed to study the effects of vacancy and pressure on the structure and initial decomposition of crystalline 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (β-NTO), a high-energy insensitive explosive. A comparative analysis of the chemical behaviors of NTO in the ideal bulk crystal and vacancy-containing crystals under applied hydrostatic compression was considered. Our calculated formation energy, vacancy interaction energy, electron density difference, and frontier orbitals reveal that the stability of NTO can be effectively manipulated by changing the molecular environment. Bimolecular hydrogen transfer is suggested to be a potential initial chemical reaction in the vacancy-containing NTO solid at 50 GPa, which is prior to the C-NO2 bond dissociation as its initiation decomposition in the gas phase. The vacancy defects introduced into the ideal bulk NTO crystal can produce a localized site, where the initiation decomposition is preferentially accelerated and then promotes further decompositions. Our results may shed some light on the influence of the molecular environments on the initial pathways in molecular explosives.

The cocrystallization effect plays a prominent role in improving the performance of energetic materials. A combinational strategy based on density functional tight-binding molecular dynamics (DFTB-MD) simulations and density functional theory (DFT) was used to elucidate the decomposition mechanisms and reaction kinetics of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)-based cocrystal explosives [HMX/N,N-dimethylformamide (DMF) and HMX/bis­(2,4-dinitro-2,4-diazapentane) (DNDAP)] at high temperatures. The decomposition and reaction mechanisms of the two cocrystals showed great dependence on the temperature. In the HMX/DMF cocrystal, a conformational change of HMX at 2000 K and subsequent initial decomposition of HMX at 2500 K is involved. At 3000 K, the global decomposition and interaction of the HMX and cocrystal molecules occurred. There are two dominant competing reaction channels in the two cocrystals. The comparative results reveal that the HMX molecules have a larger reactivity with DNDAP at low temperatures but with DMF at high temperatures. The decomposition pathways of the HMX molecules based on DFT calculations were studied as a useful addition to the MD results. These findings provide a basic understanding of the thermal decomposition mechanisms and reaction kinetics of HMX-based cocrystal energetic materials at high temperatures.

Density functional tight-binding molecular dynamics simulations with dispersion corrections were performed to study the adiabatic initial decomposition processes of molecular explosive α-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (α-HMX) nanoparticles (NPs) with the diameters of 1.4–2.8 nm at high temperatures from 2400 to 3000 K. The results indicate that the global thermal decompositions of the HMX NPs present great dependence on the temperature and particle size. The initial decomposition process of the HMX NPs includes two sequential stages: (i) competition between rapid expansion and unimolecular decompositions at surfaces; (ii) subsequent complex uni- and bimolecular decompositions. The main decomposition pathway at low temperatures is the isomerization reaction of the HMX molecule, which is quite different from the N–NO2 homolysis with ring opening at high temperatures. A second-order rate model was established to rationalize the global decompositions of the HMX NPs at different temperatures as well as the elementary decomposition paths. Our findings suggest that the thermal decompositions of the HMX NPs are quite different from their solid phase decompositions in both decomposition mechanisms and global kinetics. These findings provide basic understandings of initial decompositions of nanosized explosives.

We report the most stable packings of five HMX/solvent supramolecular assemblies.