A computational study of density of some high energy molecules

Abstract

The detonation pressure depends quadratically on the loading density of the explosives. A precise estimate of the density is thus crucial to decide if a novel energetic material is worth pursuing. In this work we investigate theoretically the crystal densities of the energetic compounds RDX, TNT, NTO, DNAM, CL-20, DADNE, and HMX. We calculate the crystal densities by using Materials Studio 7.0 Polymorph Predictor, employing force fields and exploring molecular packing arrangements with minima in total energy. Geometry optimized molecular structures computed by density functional theory (DFT) are used as input to the density predictions. In an additional DFT study we apply two functionals, B3LYP and M06 with the 6-31G(d) and the 6-31G(d,p) basis sets, and the program package GAUSSIAN09. In this part of the work crystal densities are calculated by using the molecular isosurface volume (defined by the volume within a surface with an electron density of 0.001 electrons per Bohr3) alone or combined with the variance of the electrostatic potential (ESP). The Polymorph Predictor seems to overestimate the densities, but the values are very dependent on the force field strength determined by charges assigned to atoms. In the GAUSSIAN09 DFT study the densities derived by using the M06 functional are in similar agreement with experimental data as what we experienced for the B3LYP results, although both functionals appear to give slightly lower densities than reported experimentally for the majority of the molecules. On average, the densities derived by the ESP method correlate equally well with measured values as the results obtained by the isosurface method.