THz in form II can be explained by the extra feature predicted on the high frequency side of the intense peak at 1.82 THz; (iii) the broad shoulder at 2.5 THz in the experimental spectra of the form II samples matches well with the two less intense peaks at 2.37 and 2.58 THz predicted for form II. Overall, the simulated spectra show a greater degree of difference between the two polymorphs compared to that observed in the experimental spectra. One way to explain this could be the expectation that the form II samples are intergrown with form I domains. To explore this possibility further, a small amount of the "high purity" form II sample was used to spike the bulk form I sample and vice versa. This resulted in even closer similarity between the experimental spectra. As a final observation, the anticipated structural disorder in the form II samples would be expected to result in non-coherent and less pronounced vibrational motions, which are suggested by the increasing of the baseline in the THz spectra. 31 To quantify the comparison, the experimental spectra were fitted in Matlab with the spectral features constrained to the frequencies predicted by the simulations, where the peaks were fitted to Lorentzian functions and the baseline was fitted to a power law (Eqn 1).