The relatively poor energetic rankings for ATZ and FOX-7 are likely related to the flexibility of the NH 2 groups causing mismatches between our gas-phase optimized geometry and the molecular geometry within the experimental crystal structures. For HMX, the α and δ polymorphs are ranked poorly. Both of these polymorphs contain the chair-chair HMX conformer and although α-HMX corresponds to the lowest energy predicted structure from the chair-chair conformation, it is far above the global minimum in total energy. The HMX results show that the mixed force field intermolecular energy + DFT conformational energy model does not work well in this case.