The high-resolution waveguide terahertz (THz) time-domain spectra (20-100 cm(-1)) of the two structural isomers 1 2-dicyanobenzene (1 2-DCB) and 1 3-dicyanobenzene (1 3 DCB) have been modeled and assigned using solid-state density functional theory The THz spectra of these similar molecules are distinctly different in the low-frequency region with the differences being driven by modifications of the crystal packing arrangement between the isomers Simulations utilizing the hybrid density functionals B3LYP and PBE0 were performed to determine the origins of the observed vibrational features External lattice vibrations (hindered translations and rotations) are found to dominate these spectra reinforcing the need for proper solid-state models in the analysis of the THz spectra of organic molecular solids These calculations wet e able to account for all of the observed spectral features exhibited by both Isomers even in the case of 1 2-DCB where the spectrum was found to be the result of two coexisting crystalline polymorphs