We have calculated the full dielectric function in the far-infrared for the high explosive beta-HMX using density functional perturbation theory. Complementary time-domain terahertz spectroscopy measurements were performed on military-grade samples of HMX. The calculated absorption coefficient in the range 0.1-3.6 THz is in excellent quantitative agreement with our experimentally determined absorption coefficient in terms of both peak positions and magnitude. Our calculated refractive index below 1 THz is 1.68, very close to our measured value of 1.71. This computational method appears well suited for first-principles calculation of the terahertz optical properties of molecular crystals. Published by Elsevier B. V.