Surface-Induced Energetics, Electronic Structure, and Vibrational Properties of β‑HMX Nanoparticles: A Computational Study

Surface chemistry plays an prominent part in the behaviors of condensed phase materials and nanoparticles. A combinational strategy based on density-functional theory (DFT) and density-functional tight-binding (DFTB) methods was used to study the surface-induced effect on the energetics, electronic structure, and vibrational properties of a series of β-octatetramethylene tetranitramine nanoparticles (β-HMX NPs). A comparative analysis of the NPs, isolated constituent molecule, and periodic solid-state phase of β-HMX indicates that the NPs possess quite different characteristics from either the constituent molecule or the bulk crystal. The anitsotropy of surface energies, enthalpy of sublimation, and melting point for the NPs are predicted. The surface-induced surface states of the HMX NPs lead to a significant reduction of the energy gap and provide active sites at surfaces. The vibrational properties of the experimentally determined strong modes are compared and discussed among the NPs, gas phase, and solid phase of HMX. The possible role of the surface molecules for the NPs in decreasing the material stability is elucidated. Our results provide basic understandings of the high activity of nanosized energetic materials.