Assessment of dispersion corrected atom centered pseudopotentials: application to energetic molecular crystals

A comparative study of the structural features of molecular crystals 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), cyclotrimethylenetrinitramine (RDX), 2,4,6-trinitro-1,3,5-benzenetriamine (TATB), and pentaerythritol tetranitrate (PETN) at ambient pressure were calculated using density functional theory (DFT) and dispersion corrected atom centered pseudopotentials (DCACPs). While the molecular structural parameters showed little dependence on the pseudopotential used, the overall volume and lattice vector lengths were substantially improved by the use of DCACPs. DCACP predictions of lattice vector lengths are within 1.3% of experiment, corresponding to an approximately 2% error in density. Conversely, DFT underestimates crystal densities by 5-10%. The results indicate that DCACPs substantially compensate for the inadequate description of van der Waals interactions in DFT. Additionally, comparison of DCACP predictions with analogous calculations using an alternate dispersion corrected DFT method indicate while both show an improvement over traditional DFT methods in describing van der Waals interactions for the molecular crystals in this study, the DCACP method is more accurate in predicting lattice vectors.