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RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations
journal contribution
posted on 2016-08-26, 18:10 authored by Dan C. Sorescu, Betsy M. RicePrediction of the
density and lattice compression properties of
the α and γ phases of the hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) crystal and of the low-pressure α
→ γ phase transition upon pressure increase are general
tests used to assess the accuracy of density-functional-theory- (DFT-)
based computational methods and to identify the essential parameters
that govern the behavior of this high-energy-density material under
extreme conditions. The majority of previous DFT studies have analyzed
such issues under static optimization conditions by neglecting the
corresponding temperature effects. In this study, we extend previous
investigations and analyze the performance of dispersion-corrected
density functional theory to predict the compression of RDX in the
pressure range of 0–9 GPa and the corresponding α →
γ phase transition under realistic temperature and pressure
conditions. We demonstrate that, by using static dispersion-corrected
density functional theory calculations, direct interconversion between
the α and γ phases upon compression is not observed. This
limitation can be addressed by using isobaric–isothermal molecular
dynamic simulations in conjunction with DFT-D2-calculated potentials,
an approach that is shown to provide an accurate description of both
the crystallographic RDX lattice parameters and the dynamical effects
associated with the α→ γ phase transformation.
An even more comprehensive and demanding analysis was done by predicting
the corresponding shock Hugoniot curve of RDX in the pressure range
of 0–9 GPa. It was found that the theoretical results reproduce
reasonably well the available experimental Hugoniot shock data for
both the α and γ phases. The results obtained demonstrate
that a satisfactory prediction of the shock properties in high-energy-density
materials undergoing crystallographic and configurational transformations
is possible through the combined use of molecular dynamics simulations
in the isobaric–isothermal ensemble with dispersion-corrected
density functional theory methods.