With the ability to penetrate paper, plastics, textiles, and other types of materials, light in the far-IR, or terahertz (THz), region is being increasingly studied for security and other applications. Scientists analyzing high explosives such as TNT, HMX, and RDX have reported that these compounds exhibit unique spectral fingerprints in the THz region, yet the molecular origin of the distinguishing features has remained unknown. Now, Syracuse University chemists Damian G. Allis, Darya A. Prokhorova, and Timothy M. Korter have used solid-state modeling methods to assign all of the features observed in the THz spectrum (3-120 cm-1) of the β-crystal form of HMX to distinct low-frequency vibrational motions (J. Phys. Chem. A 2006, 110, 1951). The modeling study, which may advance THz spectral analysis of pharmaceuticals and other compounds, demonstrates the shortcomings of using isolated-molecule (gas phase) theoretical methods to analyze motions in molecular solids.