The cryogenic terahertz spectrum of (+)-methamphetamine hydrochloride From 10.0 to 100.0 cm(-1) is presented. as is the complete structural analysis and vibrational assignment of the compound using solid-state density functional theory. This cryogenic investigation reveals multiple spectral features that were not previously reported in room-temperature terahertz studies of the title compound. Modeling of the compound employed eight density functionals utilizing both solid-state and isolated-molecule methods. The results clearly indicate the necessity of solid-state Simulations for the accurate assignment of solid-state THz spectra. Assignment of the observed spectral features to specific atomic motions is based oil file BP density functional, which provided the best-fit solid-state simulation of the experimental spectrum. The seven experimental spectral features are the result of thirteen infrared-active vibrational modes predicted Lit a BP/DNP level of theory with more than 90% of the total spectral intensity associated with external crystal vibrations.