## 3 Results
### Molecular Dynamics Simulations
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Structure** & **a [Å]** & **b [Å]** & **c [Å]** & \(\alpha\) [\({}^{\circ}\)] & \(\beta\) [\({}^{\circ}\)] & \(\gamma\) [\({}^{\circ}\)] & **V [Å\({}^{3}\)]** & **E [kJ/mol]** \\ \hline Phase I & 7.249 & 7.376 & 8.489 & 90.0 & 90.0 & 90.0 & 453.9 & 0 \\ Phase II & 7.460 & 7.570 & 7.680 & 90.0 & 90.0 & 90.0 & 433.7 & 14.46 \\
6 ns & 7.446 & 7.193 & 8.529 & 89.6 & 90.8 & 79.7 & 449.5 & 37.29 \\ \hline \end{tabular}
\end{table} 
Table 5: Comparison between Phase I and Phase II, and snapshot taken at 6 ns of aiMD of SA optimised at 20.1 GPa using the PBE TS method. Unit cell dimensions of Phase II were fixed to their experimental values while unit cell dimensions of Phase I and “6 ns” were optimised. All atoms positions were optimised as well for all three structures. E stands for relative energy, with reference to the energy of Phase I. The structural differences between Phase I and Phase II, as well as the molecular mechanism responsible for this transition, were already described in details in [18]. Based on the experimental results, the authors deduced the mechanism of the phase change as follows. Under ambient conditions, hydrogen bonding and electrostatic interactions are the predominant forces within SA. The transition of SA under high pressure is anticipated to arise from the cooperativity of these two interactions. As pressure escalates, the distances  
Figure 8: Length distribution of the H-bond during the aiMD simulations at \(1\) atm (blue) and \(20.1\) GPa (green). Figure 10: Length evolution of the H-bonds during the aiMD simulations at 20.1 GPa. Different colors represent the unique H-bonds within the unit cell. Figure 9: Length evolution of the H-bonds during the aiMD simulations at 1 atm. Different colors represent the unique H-bonds within the unit cell. between molecules diminish. Hydrogen bonding and electrostatic interactions between the adjacent SO3- and NH3+ groups are intensified, leading to an increase in the total Gibbs free energy. Thus, to maintain structural equilibrium, clusters of SO3- and NH3+ begin to deform beyond 8.1 GPa, accompanied by the formation of new hydrogen bonds. Upon additional compression between 10.1 and 12.7 GPa, the structure can no longer accommodate the rising free energy, prompting SA to transition to a new conformation to minimise the free energy. Pressure causes relative sliding movements between adjacent molecular chains in SA, accompanied by the reconfiguration of hydrogen bonding and the intensification of electrostatic interactions. Nevertheless, the phase change does not cause an obvious breakdown of the structure. Following the relative displacement of the molecules, the high-pressure structure retains a resemblance to the ambient structure. Despite a sudden alteration in unit cell characteristics, the high-pressure phase can maintain the same symmetry as the ambient phase. As a result, SA undergoes an isosymmetric transition at an elevated pressure. Phase II can be seen as the outcome of the relative displacement between adjacent hydrogen-bonded molecular chains of Phase I. The apparent direction of the slippage is just along the c-axis. The c-axis exhibits a significant contraction following the phase shift. Concurrently, the movement between the molecules results in a slight elongation of a- and b-axes. Moreover, in the ab plane, the angle of intersection between two SA molecules diminishes, corroborating our assertion that Phase II is more compact than Phase I. The tight structure of Phase II renders the hydrogen-bonded network challenging to compress under high pressure.