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 0.42 to 0.55 cm(3)/g, corresponding to relative volumes from 0.8 to 1.03. The calculated 300 K isotherm agrees very well with the experimentally measured pressure-volume relation. We also discuss thermodynamic properties of the material such as the volumetric thermal expansion coefficient, the Gruneisen parameter, and the specific heat (1.0 kJ/kg/K at 300 K and atmospheric pressure). The developed computational approach exhibits a reliable predictive power and is easily transferable to other molecular materials.