tributed to the α-and β -relaxations. However, it is somewhat surprising that, while the fundamental α-and β -relaxations occur at relatively low frequencies (kHz to MHz) in dielectric spectroscopy, their associated dynamics can also be observed at much higher frequencies. For example, THz-TDS covers the spectral region of 0.1 -3 THz i.e. frequencies some nine orders of magnitude greater than the fundamental α-relaxation process. There have been attempts to reconcile these observations within the context of the Coupling Model , which argues that spectral changes observed at terahertz frequencies are coupled to their fundamental relaxations via a so-called 'fast process', typically referred to as 'cage-rattling' motion . However, given that a wide range of methods all yield similar results, even though they are based on very different measurement principles and probe molecular properties over a wide range of energies, it is possible, and indeed likely, that a more fundamental feature, that is, the PES, that relates the experimental observations and atomic-level interactions. This is what is suggested in the Coupling Model, but rather than limit the application to 'cage-rattling' dynamics, we propose that the PES, both intramolecular and intermolecular, is responsible for dictating the complete intermolecular dynamics, inclusive of any cage-rattling motions. This is particularly important in the case of organic molecular-solids, as the low-frequency modetypes are often a mixture of intra-and intermolecular motions that cannot be easily separated from one another.