Density Functional Theory Studies of Energetic Materials

First-principles calculations employing density functional theory (DFT) were performed on the energetic materials PETN, HMX, RDX, nitromethane, and a recently discovered material, nitrate ester 1 (NEST-1). The aims of the study were to accurately predict the isothermal equation of state for each material, improve the description of these molecular crystals in DFT by introducing a correction for dispersion interactions, and perform uniaxial compressions to investigate physical properties that might contribute to anisotropic sensitivity. For each system, hydrostatic-compression simulations were performed. Important properties calculated from the simulations such as the equilibrium structure, isothermal equation of state, and bulk moduli were compared with available experimental data to assess the agreement of the calculation method. The largest contribution to the error was believed to be caused by a poor description of van der Waals (vdW) interactions within the DFT formalism. An empirical van der Waals correction to DFT was added to VASP to increase agreement with experiment. The average agreement of the calculated unit-cell volumes for six energetic crystals improved from approximately 9% to 2%, and the isothermal EOS showed improvement for PETN, HMX, RDX, and nitromethane. A comparison was made between DFT results with and without the vdW correction to identify possible advantages and limitations. Uniaxial compressions perpendicular to seven low-index crystallographic planes were performed on PETN, HMX, RDX, nitromethane, and NEST-1. The principal stresses, shear stresses, and band gaps for each direction were compared with available experimental information on shock-induced sensitivity to determine possible correlations between physical properties and sensitivity. The results for PETN, the only system for which the anisotropic sensitivity has been thoroughly investigated by experiment, indicated a possible correlation between maximum shear stress and sensitivity. The uniaxial compressions that corresponded to the greatest maximum shear stresses in HMX, RDX, solid nitromethane, and NEST-1 were identified and predicted as directions with possibly greater sensitivity. Experimental data is anticipated for comparison with the predictions