Michael Conroy

Density functional theory calculations of solid nitromethane under hydrostatic and uniaxial compressions with empirical van der Waals correction.

First-principles density functional theory calculations have been performed with and without an empirical van der Waals (vdW) correction to obtain constitutive relationships of solid nitromethane under hydrostatic and uniaxial compressions. The unit-cell parameters at zero pressure and the hydrostatic equation of state at 0 K are in reasonable agreement with experimental data using pure DFT, and the agreement is significantly improved with the inclusion of the vdW dispersion correction. Uniaxial compressions normal to the {100}, {010}, {001}, {110}, {101}, {011}, and {111} planes were performed, and a comparison of the principal stresses, changes in energy, and shear stresses for different compression directions clearly indicate anisotropic behavior of solid nitromethane upon compression. The calculated anisotropic constitutive relationships might help to link the anisotropic shock sensitivity and the underlying atomic-scale properties of solid nitromethane.

First-principles anisotropic constitutive relationships in β-cyclotetramethylene tetranitramine (β-HMX)

First-principles density functional theory calculations have been performed to obtain constitutive relationships in the crystalline energetic material β-cyclotetramethylene tetranitramine (β-HMX). In addition to hydrostatic loading, uniaxial compressions in the directions normal to the {100}, {010}, {001}, {110}, {101}, {011}, and {111} planes have been performed to investigate the anisotropic equation of state (EOS). The calculated lattice parameters and hydrostatic EOS are in reasonable agreement with the available experimental data. The uniaxial compression data show a significant anisotropy in the principal stresses, change in energy, band gap, and shear stresses, which might lead to the anisotropy of the elastic-plastic shock transition and shock sensitivity of β-HMX.

Hydrostatic and uniaxial compression studies of 1,3,5-triamino- 2,4,6-trinitrobenzene using density functional theory with van der Waals correction

Hydrostatic and uniaxial compressions of 1,3,5-triamino-2,4,6-trinitrobenzene were investigated using first-principles density functional theory with an empirical van der Waals correction. The equilibrium structural and elastic properties and the hydrostatic equation of state are in good agreement with available experimental data. Physical properties such as the principal stresses, shear stresses, band gap, and the change in energy per atom as a function of compression ratio V/V0 in the directions normal to the (100), (010), (001),(110), (101), (011), and (111) crystallographic planes were calculated, showing highly anisotropic behavior under uniaxial compressions.

Density functional theory calculations of anisotropic constitutive relationships in alpha-cyclotrimethylenetrinitramine

Constitutive relationships in the crystalline energetic material alpha-cyclotrimethylenetrinitramine (alpha-RDX) have been investigated using first-principles density functional theory. The equilibrium properties of alpha-RDX including unit cell parameters and bulk modulus, as well as the hydrostatic equation of state (EOS), have been obtained and compared with available experimental data. The isotropic EOS has been extended to include the anisotropic response of alpha-RDX by performing uniaxial compressions normal to several low-index planes, {100}, {010}, {001}, {110}, {101}, {011}, and {111}, in the Pbca space group. The uniaxial-compression data exhibit a considerable anisotropy in the principal stresses, changes in energy, band gaps, and shear stresses, which might play a role in the anisotropic behavior of alpha-RDX under shock loading.

APPLICATION OF VAN DER WAALS DENSITY FUNCTIONAL THEORY TO STUDY PHYSICAL PROPERTIES OF ENERGETIC MATERIALS

An empirical correction to account for van der Waals interactions based on the work of Neumann and Perrin [J. Phys. Chem. B 109, 15531 (2005)] was applied to density functional theory calculations of energetic molecular crystals. The calculated equilibrium unit‐cell volumes of FOX‐7, β‐HMX, solid nitromethane, PETN‐I, α‐RDX, and TATB show a significant improvement in the agreement with experimental results. Hydrostatic‐compression simulations of β‐HMX, PETN‐I, and α‐RDX were also performed. The isothermal equations of state calculated from the results show increased agreement with experiment in the pressure intervals studied.

HYDROSTATIC EQUATION OF STATE AND ANISOTROPIC CONSTITUTIVE RELATIONSHIPS IN 1,3,5‐TRIAMINO‐2,4,6‐TRINITROBENZENE (TATB)

Using first‐principles Density Functional Theory (DFT) with an empirical van der Waals (vdW) correction, we studied the equilibrium properties and hydrostatic equation of state (EOS) for TATB, and compared the results with experimental data. The equilibrium unit‐cell parameters and isothermal EOS calculated with vdW‐DFT show better agreement with experiment than standard DFT, which does not provide a proper description of long‐range dispersive interactions. Uniaxial compressions normal to the {001}, {010}, {011}, {100}, {101}, {110}, and {111} crystallographic planes were also studied. Calculated mechanical properties such as principal and shear stresses and the energy per atom show a clear anisotropy upon uniaxial compression.

First-principles thermodynamics of energetic materials

Using density functional theory with empirical van der Waals corrections, cold pressure curves were calculated and combined with the quasi-harmonic approximation to study thermodynamical properties of several energetic molecular solids. Vibration spectra at each compression were calculated and used for including temperature and zero-point energy contributions to the free energy. Equilibrium properties at temperatures of experiments, as well as hydrostatic equations of state, specific heat capacities, and coefficients of thermal expansion, were obtained and compared to experiment.

ANISOTROPIC CONSTITUTIVE RELATIONSHIPS IN ENERGETIC MATERIALS: PETN AND HMX

This paper presents results of first-principles density functional calculations of the equation of state (EOS) of PETN-I and beta-HMX. The isotropic EOS for hydrostatic compression has been extended to include uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] directions up to compression ratio V/V0 = 0.70. Equilibrium properties, including lattice parameters and elastic constants, as well as hydrostatic EOS are in good agreement with available experimental data. The shear stresses of uniaxially compressed PETN-I and beta-HMX have been evaluated and their behavior as a function of compression ratio has been used to make predictions of shock sensitivity of these EMs. A comparison of predicted sensitivities with available experimental data has also been performed.

FIRST‐PRINCIPLES STUDIES OF HYDROSTATIC AND UNIAXIAL COMPRESSION OF A NEW ENERGETIC MATERIAL ‐AN ENERGETIC NITRATE ESTER

Density functional theory (DFT) calculations with an empirical van der Waals (vdW) correction were performed on a new energetic material (EM), a nitrate ester that was recently synthesized by Chavez et al. [Angew. Chem. Int. Ed. 47, 8307 (2008)]. This EM was shown to have physical properties superior to another nitrate ester, PETN. The equilibrium structure was calculated by vdW‐DFT in excellent agreement with experiment (to within about 0.6% of the equilibrium volume of the unit cell). From the hydrostatic‐compression simulation, the isothermal equation of state and bulk modulus was predicted prior to any known experimental results. In addition, uniaxial compressions were simulated in the , , , , , , and Indirections to examine the anisotropic quality of the constitutive relationships. The calculated physical properties of the nitrate ester at extreme conditions are compared with other important EMs.

ANISOTROPIC CONSTITUTIVE RELATIONSHIPS IN ENERGETIC MATERIALS: NITROMETHANE AND RDX

The anisotropic constitutive relationships in solid nitromethane (NM) and α‐RDX were studied using first‐principles density functional theory (DFT). In addition to hydrostatic compressions, we performed uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] directions up to the compression ratio V/V0 = 0.70. Equilibrium properties, including lattice parameters and elastic constants, as well as hydrostatic EOS, are in good agreement with available experimental data. The shear stresses of uniaxially compressed NM and α‐RDX were used to predict the relative shock sensitivity between different crystallographic directions.