Thermal Expansion of Explosive Molecular Crystals: Anisotropy and Molecular Stacking

Molecular dynamics simulations of three typical explosive crystals, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 1,1-diamino-2,2- dinitroethene (FOX-7) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), were carried out under NPT ensemble and selected force field. The equilibrium structures at elevated temperatures were obtained, which show that the stacking behaviour of the molecules does not change with temperature. The coefficient of thermal expansion (CTE) values were calculated by linear fitting methods, and the results show that the CTE values are close to the experimental results and are anisotropic. The total energies of the cells expanding along each single crystallographic axis were calculated by the periodic density functional theory method, indicating that the energy change rates are anisotropic, and correlation equations of the energy change vs. CTE values were established. The essence of the anisotropy of the explosive crystal’s thermal expansion was compared and elucidated.