Ab initio simulation of defects in energetic materials: Hydrostatic compression of cyclotrimethylene trinitramine

An isotropic compression of both the perfect solid cyclotrimethylene trinitramine $(C3H6N6O6),$ also known as RDX, and of the solid containing vacancies is simulated using the ab initio Hartree–Fock method combined with two different crystal models: a periodic (band structure) and a molecular cluster. We show that an external pressure causes a significant decrease of the optical gap for both the perfect material and the crystal with vacancies. The solid RDX is found to be highly compressible; a pure crystal could be compressed to 57% of its equilibrium volume, whereas crystals containing vacancies are even more compressible. The critical pressure necessary for the insulator–metal transition is also predicted. It is shown that the voids present in a real RDX solid lower the metallization pressure by about 30%. Theoretical results are in close agreement with the experimental data on solid and porous RDX. The influence of defects present in the crystal and the relation to control of the sensitivity to detonation are discussed in detail.