In the terahertz region the agreement between experiment and static calculations are poor and the importance of the dynamics of the system to the resultant spectra is clear. Below 100 K, anharmonic effects and disorder are expected to be small and the agreement between the calculated spectrum (88 K based on SCI8) and the experimental one of Ren et al. is quite good, but there are some features missing. The inclusion of the disorder into the system by using the DCI32 cell in conjuction with the added benefits of the dynamics improves the correlation between the calculation and experiment. This is surprising considering that crystallography would suggest there there is much less disorder at these temperatures. The correlation could be improved further below 100 cm -1 and this could be owing to the amount of disorder at these temperatures with the DCI32 assuming a even mix of both hydrogen bonding motifs. For in-stance the peak at 50 cm -1 in the experimental spectra could be owing to some S-H...O bonding that has become frozen in on preparation of the sample as static calculations indicate that only this hydrogen bonding motif gives rise to such a low frequency absorption. Alternatively it could be owing to the boson peak which has been identified in the low temperature Raman spectrum at 40 K, which was associated with disorder of the small amount of water in the sample. Comparisons between experiment and theory is further complicated by light scattering from air voids in the sample which is known to give rise to a background rise in absorption and has not been accounted for here. A compelling reason for performing MD calculations for the calculation of IR spectra is to capture the anharmonic effects that maybe expected. Our results here show that the MD calculations are very useful below about 300 cm -1 where they capture features that static calculations do not. Above this threshold the agreement between MD and static is very good and probably does not justify the computational expense of the MD calculation. Below this threshold and especially below 150 cm -1 there are significant differences in the comparisons between the methods, in particular the static methods predict lower absorption than that predicted by MD. The reason for this is not clear as at higher frequencies the absorption is very similar using either method. One possibility could be that the dipole moment derivatives found through MD are enhanced by being in an anharmonic region. A similar idea was put forward by Bordallo et al. who suggested that once the rotational motions became free there would be a strong coupling with the phonons giving rise to much larger amplitude motion. But the simulation of Form I at 88 K using MD still showed an enhanced absorption at low frequencies and the motion at such a low temperature is expected to be harmonic where the methyl rotors are not free to rotate.