## 3 Results
### Geometry Optimisation of SA at 1 atm and 20.1 GPa
Additionally, although the authors of [18] did not deposit the crystal structure of Phase II, they compared both phases in Figure 9 of [18], revealing that no major changes in the molecular structure occurred during IPT. Finally, as during geometry optimisation, all atom positions were relaxed, and molecular properties (distance, angles, torsions) were optimised to reach the lowest enthalpy conformation. Then, the energy of Phase I and Phase II was compared (Table 4). Analysis of the results presented in Table 4 revealed that at 20.1 GPa Phase I was energetically preferable (\(\Delta\)E > 0). Since the results of the DFT calculations may depend on the choice of the DFT functional, a large variety of methods (functional + dispersion correction) were used in this study to validate if the sign of \(\Delta\)E will depend in the choice of the method. The positive value of \(\Delta\)E between Phase II and Phase I of dozen kJ/mol was obtained, regardless of the method used. It was confirmed that functional/dispersion choice does not alter the conclusion that Phase I is higher in energy. This explained why the optimisation of Phase I at 20.1 GPa did not result in the phase transition and why the \begin{table}
\begin{tabular}{c c c} \hline \hline
**DFT Functional** & **Dispersion** & \(\Delta\)**E Phase II—Phase I** \\  & **Correction** & **[k/mol]** \\ \hline PBE & TS & 14.46 \\ PBE & Grimme & 13.33 \\ PBE & MBD* & 12.73 \\ PBE & n/a & 16.74 \\ RPBE & TS & 5.75 \\ RPBE & n/a & 26.69 \\ PW91 & TS & 14.03 \\ PW91 & OBS & 14.03 \\ PW91 & n/a & 16.14 \\ WC & n/a & 12.03 \\ PBESOL & TS & 11.36 \\ PBESOL & n/a & 12.22 \\ BLYP & TS & 8.50 \\ BLYP & Grimme & 16.23 \\ BLYP & n/a & 24.59 \\ RSCAN & n/a & 11.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Energy differences between Phase II and Phase I of SA optimised at 20.1 GPa. Unit cell dimensions of Phase II were fixed to their experimental values, while unit cell dimensions of Phase I were optimised. optimisation of Phase II at 20.1 GPa ended with its transformation to Phase I, which is in disagreement with the experimental results. The comparison of the molecular and crystal structures of Phase I and Phase II is presented in Figure 3. No major differences were observed between the molecular structures of SA zwitterions, and the values of angles were preserved while the differences in the bond lengths were lower than 0.02 A (Figure 3A). In both forms, the SA crystal structure was stabilised by a dense network of hydrogen bonds. The comparison of the H-bonding between Phase I and Phase II (Figure 3C) revealed that while the number of hydrogen bonds was preserved, their lengths were altered. Some H-bonds of Phase II were longer (i.e., 1.680 A vs. 1.580 A) while the other were shorter (i.e., 2.003 A vs. 2.265 A) than the corresponding ones of Phase I. Moreover, the phase transition resulted in alterations in the intermolecular S-N distances between the neighboring molecules (Figure 3D). While some of those S-N distances remained unchanged (5.280 vs. 5.283 A), the other ones were significantly shortened (i.e., 3.575 vs. 3.799 A). This confirms that transition results from anisotropic compression and sliding of the molecules among the specific direction. Overall, Phases I and I are clearly different from each other (Figure 3B).