## 3 Results
### Raman Spectra Calculations and Comparison with Experiment  
Although the geometry optimisation of Phase I at 20.1. GPa did not result in the anticipated IPT, this did not discourage us from continuing our research. Since apart from XRD Raman spectrometry was also used to describe the IPT of SA [18], we decided to calculate the Raman spectra of Phase I at 1 atm and 20.1 GPa, as well as Phase II at  
Figure 3: Comparison of the molecular and crystal structures of Phases I and II optimised at 20.1 GPa, showing the structural differences resulting from polymorphic transitions. (**A**) Comparison of bond lengths; (**B**) superimposed unit cells; (**C**) comparison of H-bonding network and lengths of the H-bonds present in the unit cells; (**D**) comparison of the intermolecular S-N distances. 20.1 GPa. Here, it should be noted that in order to obtain the Raman spectra of Phase II at 20.1 GPa, we could not optimise the unit cell dimensions, as it resulted in its transition to Phase I (Table S1). Therefore, for that purpose, we performed the optimisation of Phase II at 20.1 GPa with the constrained unit cell dimensions using the same approach described above (Table 4). Then, the calculated (at 1 atm and 20.1 GPa) Raman spectra were compared with the experimental ones, recorded in the pressure range of 1 atm-18.9 GPa (Figure 4). Figure 4: Comparison of experimental (**top**) (recorded in the pressure range 1 atm-18.9 GPa) and theoretical (**bottom**) (calculated at 1 atm and 20.1 GPa) Raman spectra of SA. Reprinted from [18], with the permission of AIP Publishing. Analysis of the spectra recorded and calculated at 1 atm, within the 1400-1600 cm\({}^{-1}\) range, revealed the presence of the low-intensity overlapping peaks at 1440 cm\({}^{-1}\). Also, for both experimental and theoretical spectra, four peaks were observed in the 1540-1580 cm\({}^{-1}\) range. In the 2900-3300 cm\({}^{-1}\) region, the peaks in the experimental spectrum were more overlapping; however, both for experimental and theoretical spectra, few peaks were observed, with the most intensive one at c.a. 3050 cm\({}^{-1}\). Overall, good agreement between the experimental and calculated Raman spectra of Phase I at 1 atm was observed, with the differences between the corresponding peaks lower than 50 cm\({}^{-1}\). Analysis of the theoretical spectra of Phase I and Phase II at 20.1 GPa revealed some major differences among them. In the 1400-1600 cm\({}^{-1}\) range, in the theoretical spectrum of Phase II, a peak can be observed at 1455 cm\({}^{-1}\), which was absent in the spectrum of Phase I. This peak was also observed in the experimental spectra recorded at a higher pressure, and was associated by the authors of [18] with the formation of the new Phase II. Additionally, the theoretical spectra of Phase I and II differed substantially in the 2900-3250 cm\({}^{-1}\) region. For example, solely in the spectrum of Phase I, the intensive peak was observed at 2950 cm\({}^{-1}\). This peak was observed experimentally at 1 atm, but disappeared at a higher pressure due to the phase transition. In the experimental spectrum, the overlapping peaks were observed in the 3000-3200 cm\({}^{-1}\) region. Also, a couple of peaks in the theoretical spectrum of Phase II were present in this range. On the contrary, the peaks in the theoretical spectrum of Phase I at 20.1 GPa were characterised by lower wavenumbers in the range of 2920-3200 cm\({}^{-1}\). In conclusion, the comparative analysis of the experimental and theoretical Raman spectra revealed the good agreement between the experimental spectrum recorded at a higher pressure and spectrum of Phase II calculated at 20.1 GPa, with the differences between the corresponding peaks being lower than 50 cm\({}^{-1}\). This confirmed that the modelled structure of Phase II at 20.1 GPa was observed experimentally, but the questioned remained on the stability of this form, taking into account its higher lattice energy, in comparison to Phase I (Table 4).