## 3 Results
### Molecular Dynamics Simulations  
Molecular dynamics (MD) simulations can be very useful in the computational analysis of complex phenomena such as polymorphic phase transitions. The kinetic energy, associated with the non-zero T, added to the system, may allow for overcoming the potential energy barriers between the forms. Moreover, it allows for including the entropic contributions, which are neglected at 0 K.  
To check the dynamic stability of SA, we conducted MD simulations at both 1 atm and 20.1 GPa using the Phase I as the initial (Figure 6). At 1 atm, no major changes in the unit cell dimensions were being observed, apart from oscillations resulting from lattice dynamics. The average values of the unit cell lengths were close to those obtained from geometry optimisation and simultaneously to the recorded experimental values. However, at 20.1 GPa, the increase in the amplitude of the oscillations was being observed. Additionally, at c.a. 6 ns of simulation time, the significant increase in the lengths of \"a\" and \"b\" with the simultaneous decrease in \"c\" was being observed, which indicated the IPT and formation of Phase II (Figure 2). Nevertheless, Phase II was only present during the short period of the simulation time, and the backward transition to Phase I was being observed at the end of simulation. Figure 5: Differences (Phase I-Phase II) between the thermodynamic parameters: Gibbs free energy (\(\Delta\)G), enthalpy (\(\Delta\)H), and temperature times entropy (TAS) of the structures modelled using PBE TS functional at 20.1 GPa, with respect to the temperature. Figure 6: Evolution of the unit cell parameters during the aiMD simulations at 1 atm (**left**) and 20.1 GPa (**right**). As the unit cell dimensions at 6 ns of aiMD simulation resembled those of Phase II, we used the snapshot from the trajectory, recorded at 6 ns, and optimised it at 20.1 GPa. The results were compared with those obtained for Phase I and Phase II using the same computational method (Table 5). Optimisation of the 6 ns structure resulted in the local minimum, as its energy was higher than those of Phase I and Phase II. Moreover, the space group of 6 ns was not preserved and changed from _Pbca_ to P1. As the intramolecular H-bonds between the -NH\({}_{3}^{+}\) groups and oxygen atoms were the main forces stabilising the structure of SA (Figure 7), we analysed the length distribution (Figure 8) and length evolution (Figures 9 and 10) of the representative bond during the MD simulations at the two studied pressures (1 atm and 20.1 GPa). Analysis of the length distribution at 1 atm (Figure 8) indicates the presence of two peaks. First, more intensive, confirms the presence of strong, stable H-bond, with an approximate length of 1.7 A. During the MD simulations the rotation of the -NH\({}_{3}^{+}\) group is being observed, it is possible for the H atoms of a particular group to switch positions (i.e., 2-4 ns, Figure 9), which results in the larger distance of the tracked H atom from the O atom, and consequently, the presence of the second peak at 3.0 A is shown in Figure 8.  
The analysis of the H-bonds during the simulation at 20.1 GPa reveals their completely different behaviour. The reasonable distance, for an H-bond, was observed only during the initial stage of simulation (Figure 10), while during the rest of it, the observed -NH\({}_{3}^{+}\)-O distance increased and fluctuated. This indicates that the H-bonds were being broken, which resulted in the experimentally observed IPT. Figure 7: Fragment of the crystal structure of Phase I optimised at 1 atm. H-bond (black–blue dashed line, 1.649 Å), and the distances between the other two H atoms of -NH\({}_{3}^{+}\) group and oxygen atom from the neighbouring molecule. The H-bond length analysis during the aiMD simulations is presented in Figures 8–10.