Figure shows the experimental spectra of form I and the two form II samples at 80 K. The overall resemblance of all parts of the spectra reflects the close structural similarity between the two forms, which is not generally common in other polymorphic systems. In order to facilitate comparison, the spectra were normalised relative to the peak at 1.8 THz, which is a pronounced feature in both forms (Figure ). The reason for choosing this feature will be explained in the following section of computational results. In general, the peaks of form II appeared relatively broader than those of form I, which may reflect the inherent disorder expected for form II. The three main differences are pointed out in Figure