# RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations

## Abstract

Prediction 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 paper, 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 analysismore »

- Authors:

- National Energy Technology Lab. (NETL), Pittsburgh, PA (United States)
- Army Research Lab., Aberdeen Proving Ground, MD (United States)

- Publication Date:

- Research Org.:
- National Energy Technology Lab. (NETL), Pittsburgh, PA (United States); Army Research Lab., Aberdeen Proving Ground, MD (United States)

- Sponsoring Org.:
- USDOE; USDOD

- OSTI Identifier:
- 1477233

- Resource Type:
- Journal Article: Accepted Manuscript

- Journal Name:
- Journal of Physical Chemistry. C

- Additional Journal Information:
- Journal Volume: 120; Journal Issue: 35; Journal ID: ISSN 1932-7447

- Publisher:
- American Chemical Society

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 74 ATOMIC AND MOLECULAR PHYSICS

### Citation Formats

```
Sorescu, Dan C., and Rice, Betsy M.
```*RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations*. United States: N. p., 2016.
Web. doi:10.1021/acs.jpcc.6b06415.

```
Sorescu, Dan C., & Rice, Betsy M.
```*RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations*. United States. doi:10.1021/acs.jpcc.6b06415.

```
Sorescu, Dan C., and Rice, Betsy M. Fri .
"RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations". United States. doi:10.1021/acs.jpcc.6b06415. https://www.osti.gov/servlets/purl/1477233.
```

```
@article{osti_1477233,
```

title = {RDX Compression, α→ γ Phase Transition, and Shock Hugoniot Calculations from Density-Functional-Theory-Based Molecular Dynamics Simulations},

author = {Sorescu, Dan C. and Rice, Betsy M.},

abstractNote = {Prediction 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 paper, 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. Finally, 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.},

doi = {10.1021/acs.jpcc.6b06415},

journal = {Journal of Physical Chemistry. C},

issn = {1932-7447},

number = 35,

volume = 120,

place = {United States},

year = {2016},

month = {8}

}