where 𝐴 ! is the amplitude of the peak, 𝑔 ! is the peak width, 𝑓 " ! is the predicted frequency of the peak, d is the relative shift between the simulated active modes, and 𝐵𝑥 & + 𝑐 was used to fit the baseline. Application of the shift d is common practice and is mainly due to the difference in temperature between the experimental (80 K) and simulated (0 K) results as well as additional effects of anharmonicity. The fits are shown in Figure , and the resulting parameters are shown in Table . The highlighted features in both samples are not predicted in the corresponding form, but rather in the other polymorph. The fact that it is possible to fit the experimental spectra very closely using this methodology, but that both spectra contain residual features indicative of the other polymorphic form, substantiates the hypothesis that the two polymorphs are intergrown in the samples analysed, and furthermore demonstrates that this feature can be detected by the THz measurements. It is also notable that the baseline fitting parameters of form II in Table are larger than those required for form I, consistent with the expectation that the form II sample is more disordered. This can also be reflected from the wider peak width in form II on average, as peak broadening is a significant effect to the terahertz absorption spectra caused by disorder in crystalline materials. that the first peak of form II at 1.0 THz is not fitted as well as the other features. Therefore, further calculations were performed to examine the energies of the structures with the atoms displaced in steps along the eigenvectors corresponding to the first spectral features of each form. Figure shows that the difference between the energy calculated with the harmonic approximation and the real total energy of form II was much larger than that of form I. This implies that the influence of anharmonicity is much larger for the first peak of form II than for form I, which can account for the less effective fitting that is observed. In addition, it was found that the methyl groups (-CH3) in form II were much more active than in