HMX was the only EM in the set with more than one conformer present within the known polymorphs. The conformations differ in the geometry of the eight-membered ring and are sometimes referred to as chair (β, ϵ polymorphs) and chair-chair (α, δ) in discussions of HMX polymorphism. For HMX, CSP was performed on the chair and chair-chair conformers, both of which were optimized as described above. In the force field evaluation studies, these CSPs were treated separately due to the large conformational energy difference which exists between the two conformers of HMX. For the force field results, we compare total relative energies, calculated from the sum of intermolecular energies and relative conformational energies.