## 3 Results  
### Geometry Optimisation of SA at 1 atm and 20.1 GPa  
The first stage of this work consisted of geometry optimisation, including the all atom positions and unit cell dimensions of SA Phase I at 1 atm and 20.1 GPa. Optimisation at 1 atm was performed for two reasons: first, to perform the benchmark of the available functionals (Table 1), by comparing the unit cell dimensions of calculated and experimentally recorded crystal structures; and second, to choose the most accurate method for the further calculations such as phonon DOS and aiMD. It is well known that the choice of the DFT functional can significantly affect the final results, both in non-periodic and periodic calculations. The obtained results (Table 2) confirm this statement, revealing major differences among the results obtained using various approaches. For example, the calculated unit cell dimensions differ by more than 1A, a: 9.222 A for RPBE and 7.993 A for PBESOL, which is a significant difference as it corresponds to more than 10% of the experimentally determined value. Also, some functionals (i.e., RPBE, BLYP) significantly overestimated the unit cell dimensions; however, adding the dispersion correction generally increased the accuracy of the results. The meta-GGA functional, RSCAN is located at the higher rung of the so-called Jacob's ladder of the density functional theory, with each rung generally leading to approximations with better accuracy. However, in this case, the values obtained using RSCAN were found to be underestimated. The accuracy of the PBESOL functional, designed to improve the results obtained for the solid materials, was lower than original PBE. Interestingly, in most of the cases, the calculated \"c\" unit cell length was larger than the experimentally obtained one, being also the one that undergoes rapid decrease upon IPT (Figure 2). The second step of this work was the geometry optimisation ad 20.1 GPa using the experimentally obtained crystal structure of Phase I as the initial structure. According to the experimental results [18], Phase I does not exist at this pressure; therefore, the phase transition of Phase I into Phase II during the geometry optimisation was anticipated, as observed in our earlier works [37; 38]. However, no such transition was observed this time, regardless of the method used (Table 3). The experimentally determined \"c\" length of Phase II at 20.1 GPa was 7.68 A, while in the calculations, the values higher by about 1.0 A (13%) were obtained. However, the obtained values were not just the inaccurate predictions of Phase II. A comparative analysis of Figure 2 and Table 3 indicated that the obtained values (a = 7.249 A, b = 7.376 A, c = 8.489 A, V = 453.9 A\({}^{3}\) for PBE TS, Table 3) were close to those that would be characterising Phase I if it did not undergo the IPT, and would have been stable at this elevated pressure, assuming that the compression of the unit cell dimensions continued, as modelled in Figure 2 (a = 7.5 A, b = 7.7 A, c = 8.8 A, V = 508 A\({}^{3}\)). These results could indicate two possible scenarios. First, the energy barrier between Phase I and II is too large to be overcome during the geometry optimisation [37; 38]. Second, the experimentally observed Phase II is, for some reason, not a DFT minimum at 20.1 GPa. Therefore, to verify these hypotheses, calculations of geometry optimisation at 20.1 GPa were repeated, but this time using the Phase II as the initial structure (Table S1). No