Electronic structure calculation of the structures and energies of the three pure polymorphic forms of crystalline HMX

The molecular structures and energetic stabilities of the three pure polymo rphic forms of crystalline HMX were calculated using a first-principles ele ctronic-structure method. The computations were performed using the local d ensity approximation in conjunction with localized "fireball" orbitals and a minimal basis set. Optimized cell parameters and molecular geometries wer e obtained, subject only to preservation of the experimental lattice angles and relative lattice lengths. The latter constraint was removed in some ca lculations for beta-HMX. Within these constraints, the comparison between t heory and experiment is found to be good. The structures, relative energies of the polymorphs, and bulk moduli are in general agreement with the avail able experimental data.