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

Abstract

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 (R-and γ-RDX phases), 1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane (β-, R-, 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 [R-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.

Experimental and DFT-D Studies of the Molecular Organic Energetic Material RDX

“…The observed -RDX polymorph in this study would be overlooked in such a screening, highlighting that care must be taken when choosing selection criteria in some polymorph prediction studies. agreement with a previous computational DFT-D study, 26 and are in excellent agreement with experiment over the pressure range. 60 Lattice parameters are calculated to lie within 1.6 % and 1.1 % (for -RDX and -RDX respectively) of the experimental values determined by Oswald et al 60 and all unit-cell volumes for -RDX are calculated to within 1.3 % of experiment, with the largest deviation from the experimental -RDX volume being just 0.9 %.…”

“…24 Similarly, Sorescu et al performed theoretical predictions of the responses of the crystallographic lattice parameters to pressure for ten energetic molecular crystals, including the -and -forms of RDX. 26 They concluded that the dispersion-corrected density functional theory method (DFT-D) as parameterized by Grimme 25 In this work we demonstrate the capability of the DFT-D functional parameterized by Grimme 25 to simulate accurately not just the structure, but also the vibrational and thermochemical properties of selected polymorphs of RDX. Furthermore, we provide the first experimental determination of the enthalpy of fusion of the highly metastable -form of RDX, which provides further validation of the predictive capability of the computational method used in this study.…”

“…DFT-D hydrostatic compression studies included the -and -forms of RDX, with structures produced that were in good agreement with experiment, 60 and a previous DFT-D study. 26 This comprehensive study of selected polymorphic forms of crystalline RDX at ambient and applied high-pressures has shown that the DFT-D model performs extremely well over a range of conditions, and is able to describe accurately intramolecular and intermolecular interactions and thus the structure and properties of the organic molecular crystal RDX.…”

A DFT study on structural, vibrational properties, and quasiparticle band structure of solid nitromethane

“…21 The anisotropic nature of solid nitromethane was studied using pseudopotential approach by applying hydrostatic pressure up to 20 GPa. 16 Recently, Sorescu et al 22 and Landerville et al 23 reported the structural properties of nitromethane using respectively the parametrization of Grimme 24 and of Neumann and Perrin. 25 Accurate and reliable calculations of electronic properties of secondary explosive materials with standard Density Functional Theory(DFT) are a difficult and challenging issue due to their complex crystal structure mainly controlled by van der Waals interactions.…”

Structural, elastic, optical properties and quasiparticle band structure of solid cyanuric triazide

“…From earlier reports, standard DFT methods such as local (LDA-CAPZ) and semi-local (GGA-PBE) exchange and correlation functionals are inadequate to account for the non-local van der waals interactions in these materials. For this, we have used a correction proposed by Grimme [19], which is found to work well for other energetic materials [27,28]. Hence, in the first step, we tested the accuracy of the present calculations by comparing the ground state structural properties with the experimental data.…”