Ab initio equation of state of the organic molecular crystal: beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine.

We apply a simple strategy for calculating from first principles a thermodynamically complete equation of state for molecular crystals using readily available quantum chemistry techniques. The strategy involves a combination of separate methods for the temperature-independent mechanical compression and the thermal vibrational contributions to the free energy. A first principles equation of state for beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX) has been calculated for temperatures between 0 and 400 K and for specific volumes from 0.

Ab initio studies of crystalline nitromethane under high pressure.

We have studied the mechanical compressibility and band structure of solid nitromethane both in equilibrium and compressed states using Hartree-Fock and density functional theory (DFT) with atom-centered all-electron linear combination of atomic orbitals basis sets. Hartree-Fock calculations with a 6-21G basis set, uncorrected for basis set superposition error, gave the best agreement with experimental compression studies. These results may be due to the cancellation of basis set superposition error with dispersion force errors.

Ab initio equation of state of an organic molecular crystal: 1,1-diamino-2,2-dinitroethylene.

A complete equation of state for the molecular crystal 1,1-diamino-2,2-dinitroethylene has been calculated from first principles for temperatures between 0 and 400 K, and for specific volumes from 61 to 83 cm3/mol, corresponding to relative volumes from 0.78 to 1.06.

First principles calculation of the mechanical compression of two organic molecular crystals.

The mechanical compression curves for the organic molecular crystals 1,1-diamino-2,2-dinitroethylene and beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX) are calculated using the Hartree-Fock approximation to the solutions of the many-body Schrödinger equation for a periodic system as implemented in the computer program CRYSTAL. No correction was made for basis set superposition error. The equilibrium lattice parameters are reproduced to within 1% of reported experimental values.

Equation of state and structural changes in diaminodinitroethylene under compression.

Structural changes in 1,1-diamino-2,2-dinitroethylene (DADNE, FOX-7) compressed to high pressure in diamond anvil cells were investigated using angle-dispersive x-ray diffraction analysis, Raman spectroscopy, and optical polarizing microscopy. The x-ray results show several changes above 1 GPa. When the x-ray data are indexed according to the ambient-pressure structure, DADNE shows anisotropic compression, with higher compression along the b axis than along the a or c axis.