The far-infrared spectra of the aromatic hydrocarbons tetracene and pentacene have been calculated using a perturbational-variational method coupled with potential truncation. This shortening of the potential enabled accurate vibration calculations for these large molecules and their respective dimers. Thus, it was possible to identify all IR bands obtained experimentally in the far-IR range by Michaelian et al., as well as to differentiate bands resulting from intermolecular and intramolecular modes of vibration separately, and combined intermolecular + intramolecular vibration modes. Far-IR spectra for smaller acene family members, naphthalene and anthracene, were also computed, and trends in intermolecular vibrations, for the acene family as a whole, were identified. The results obtained illustrate the quality and the detail of the insights realized by interrogating experimental spectra using high-precision, unscaled quantum mechanics computational approaches, and provide a benchmark for future work targeting identification of dominant molecular motifs and intermolecular association phenomena arising in ill-defined hydrocarbons including asphaltenes based on IR and Raman spectral decomposition.