First-principles study of high-pressure behavior of solid beta-HMX.

A first-principles plane-wave method with an ultrasoft pseudopotential scheme in the framework of the generalized gradient approximation (GGA) was used to calculate the lattice parameters, bulk modulus and its pressure derivative, energy band structures, density of states, phonon density of states, thermodynamic properties, and absorption spectra of solid beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX). The current study is focused on the thermodynamics and electronic properties that were not reported previously. The bulk modulus and its pressure derivative are also consistent with experimental data and other theoretical results. From the results for the band gaps and density of states, it was found that beta-HMX has the tendency to become a semiconductor with increasing pressure. As the temperature increases, the heat capacity, enthalpy, product of temperature and entropy, and Debye temperature all increase, whereas the free energy decreases. The optical absorption coefficients shift to higher frequencies/energies with increasing pressure. The present study leads to a better understanding of how energetic materials respond to compression.