Theoretical Predictions of Energetic Molecular Crystals at Ambient and Hydrostatic Compression Conditions Using Dispersion Corrections to Conventional Density Functionals (DFT-D)

Theoretical predictions of the crystallographic properties of a series of 10 energetic molecular crystals have been done using a semiempirical correction to account for the van der Waals interactions in conventional density functional theory (termed DFT-D) as implemented in a pseudopotential plane-wave code. This series contains compounds representative for energetic materials applications, that is, hexahydro-1,3,5-trinitro-1,3,5-s triazine (α- and γ-RDX phases), 1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane (β-, α-, and δ-HMX phases), 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL20) (ε-, β-, and γ-HNIW phases), nitromethane (NM), trans-1,2,-dinitrocyclopropane, 1,2,3,5,7-pentanitrocubane (PNC), pentaerythritol tetranitrate (PETN), 2,4,6-trinitro-1,3,5-benzenetriamine (TATB), 2,4,6-trinitrotoluene (TNT-I phase), and 1,1-diamino-2,2-dinitroethylene (FOX-7), systems belonging to diverse chemical classes that encompass nitramines, nitroalkanes, nitroaromatics, nitrocubanes, nitrate esters, and amino-nitro derivatives. At ambient pressure, we show that the DFT-D method is capable of providing an accurate description of the crystallographic lattice parameters with error bars significantly lower than those obtained using conventional DFT. Practically, for all crystals considered in this study the predicted lattice parameters are within 2% from the corresponding experimental data [α-RDX (1.58%), β-HMX (0.64%), ε-HNIW (1.42%), NM (0.75%), DNCP (1.99%), TATB (1.74%), TNT-I (0.92%), PNC(0.78%), PETN(1.35%), FOX-7(1.57%)], with the best level of agreement being found for systems where experimental data have been collected at low temperatures. A similar good agreement of the predicted and experimental crystallographic parameters was obtained under hydrostatic compression conditions as demonstrated for the cases of RDX, HMX, CL20, NM, TATB, and PETN crystals. These results indicate that the DFT-D method provides significant improvements for description of intermolecular interactions in molecular crystals at both ambient and high pressures relative to conventional DFT. In this last case, large errors of the predicted lattice parameters have been found at low pressures; theoretical values approach the experimental results only at pressures in excess of 6 GPa.