Quantum Mechanical Predictions of Energetic Materials: When Good Theories Go Bad

The performance of density functional theories (DFT) in predicting structural parameters for six conventional energetic materials (EM) over various degrees of compression was examined for a wide range of pressures. The systems studied were nitromethane, 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), cyclotrimethylenetrinitramine (RDX), 2,4,6,8,10,12hexanitrohexaazaisowurzitane (CL-20), 2,4,6-trinitro1,3,5-benzenetriamine (TATB), and pentaerythritol tetranitrate (PETN). Dependencies of results on basis set size, k-points, choice of pseudopotential and method were explored. The results indicate that at zero compression, DFT is not adequate to describe crystallographic parameters for the systems under study. However, at compressions consistent with 6 GPa or greater, DFT predictions of crystal volumes are within 5% of experiment, and are insensitive to method, choice of pseudopotential and basis set size. The results suggest that the major source of error in DFT calculations applied to systems similar to these are due to inadequate treatment of van der Waals forces, which are the dominant forces in molecular organic crystals at the ambient state.