Interest in terahertz spectroscopy of organic compounds has been driven by its potential utility as a stand-off detection technology for explosives. Much work has focused on identifying the spectral signatures of explosives compounds and understanding the molecular origin of the complex, low-frequency modes in the terahertz regime. We present distinct absorption spectra in the terahertz region for polymorphs of the secondary high explosive 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). For alpha-HMX, we couple experiments to solid-state density functional perturbation theory (DFPT) calculations, and find excellent agreement. This work underlines the importance of careful sample preparation when examining spectra and comparing experimental work with calculations. Published by Elsevier B.V.