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)

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.