A first-principles investigation of the equation of states and molecular weak spots of $beta$-cyclotetramethylene tetranitramine (HMX)

We investigate the equation of states of the $beta$-polymorph of cyclotetramethylene tetranitramine (HMX) energetic molecular crystal using DFT-D2, a first-principles calculation based on density functional theory (DFT) with van der Waals (vdW) corrections. The atomic structures and equation of states under hydrostatic compressions are studied for pressures up to 100 GPa. We found that the N-N bonds along the minor axis of the ring are more sensitive to the variation of pressure, which indicates that they are potential weak spots in atomic level within a single molecule of $beta$-HMX. Our study suggested that the van der Waals interactions are critically important in modeling this molecular crystal.