Experimental high-resolution waveguide terahertz (THz) time-domain spectroscopy and molecular modeling were used to investigate the nature of infrared active harmonic normal modes observed in solid-state organic molecules. The enhanced spectral resolution gained from using waveguide THz spectroscopy provided an opportunity to rigorously test solid-state density functional theory (DFT) calculations as a means for simulating ans assigning the THz spectra of the molecular solids. Solid-state DFT using periodic boundary conditions was used to simulate crystalline solids. The periodic calculations include the relevant molecular environment found in organic solids which cannot be replicated in isolated-molecule calculations. Several different functionals and basis sets were simulated in order to determine the best overall reproduction of the experimentally determined structure and THz frequencies. In all cases, excellent spectral agreement to the waveguide THz spectra was found.