## 3 Results
### Geometry Optimisation of SA at 1 atm and 20.1 GPa
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**CCD Ref. Code/** & **Dispersion** & & & & \\
**DFT Functional** & **Correction** & **a [Å]** & **b [Å]** & **c [Å]** & **V [Å\({}^{3}\)]** \\ \hline ICSD\_94 [35] & & 8.066(2) & 8.115(2) & 9.248(2) & 605.3(5) \\ ICSD\_25799 [36] & & 8.066(1) & 8.115(1) & 9.255(3) & 605.8(2) \\ ICSD\_803 [17] & & 8.034(1) & 8.020(1) & 9.236(2) & 595.1(2) \\ ICSD\_802 [17] & & 8.036(3) & 8.025(3) & 9.236(5) & 595.6(5) \\ PBE & TS & 8.090 & 8.089 & 9.444 & 618.0 \\ PBE & Grimme & 8.041 & 8.058 & 9.377 & 607.6 \\ PBE & MBD\({}^{*}\) & 8.118 & 8.084 & 9.417 & 618.0 \\ PBE & n/a & 8.371 & 8.188 & 9.6878 & 664.0 \\ RPBE & TS & 8.093 & 8.068 & 9.370 & 611.8 \\ RPBE & n/a & 9.222 & 8.482 & 10.137 & 792.9 \\ PW91 & TS & 8.094 & 8.091 & 9.446 & 618.6 \\ PW91 & OBS & 8.096 & 8.093 & 9.442 & 618.6 \\ PW91 & n/a & 8.374 & 8.171 & 9.689 & 663.0 \\ WC & n/a & 8.019 & 7.985 & 9.494 & 607.9 \\ PBESOL & TS & 7.933 & 7.924 & 9.282 & 583.5 \\ PBESOL & n/a & 7.993 & 7.971 & 9.407 & 599.3 \\ BLYP & TS & 8.090 & 8.018 & 9.144 & 593.1 \\ BLYP & Grimme & 8.083 & 8.109 & 9.363 & 613.7 \\ BLYP & n/a & 8.660 & 8.340 & 9.813 & 708.7 \\ RSCAN & n/a & 7.983 & 8.016 & 9.327 & 596.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of experimentally determined (blue) and geometrically optimised unit cell dimensions of SA Phase I at 1 atm. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**DFT Functional** &
\begin{tabular}{l} **Dispersion** \\ **Correction** \\ \end{tabular} & **a [Å]** & **b [Å]** & **c [Å]** & **V [Å\({}^{3}\)]** \\ \hline Experimental Phase II, & & & & & \\ determined at 20.1 GPa & & 7.46 & 7.57 & 7.68 & 433.7 \\ from [18] & & & & & \\ PBE & TS & 7.249 & 7.376 & 8.489 & 453.9 \\ PBE & Grimme & 7.213 & 7.364 & 8.510 & 452.0 \\ PBE & MBD\({}^{*}\) & 7.227 & 7.365 & 8.474 & 451.1 \\ PBE & n/a & 7.268 & 7.402 & 8.548 & 459.8 \\ RPBE & TS & 7.167 & 7.315 & 8.310 & 435.6 \\ RPBE & n/a & 7.356 & 7.493 & 8.648 & 476.6 \\ PW91 & TS & 7.248 & 7.369 & 8.488 & 453.3 \\ PW91 & OBS & 7.248 & 7.369 & 8.488 & 453.4 \\ PW91 & n/a & 7.261 & 7.394 & 8.538 & 458.4 \\ WC & n/a & 7.170 & 7.304 & 8.446 & 442.3 \\ PBESOL & TS & 7.152 & 7.284 & 8.420 & 438.7 \\ PBESOL & n/a & 7.167 & 7.304 & 8.452 & 442.5 \\ BLYP & TS & 7.166 & 7.298 & 8.380 & 438.3 \\ BLYP & Grimme & 7.267 & 7.410 & 8.548 & 460.4 \\ BLYP & n/a & 7.362 & 7.476 & 8.596 & 473.1 \\ RSCAN & n/a & 7.195 & 7.336 & 8.432 & 445.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the experimentally determined (blue, a = 7.46 Å, b = 7.57 Å, c = 7.68 Å, V = 433.7 Å\({}^{3}\)) and geometrically optimised unit cell dimensions of SA at 20.1 GPa. significant differences were observed, indicating that the choice of either Phase I or Phase II initially for calculations had no influence on the results received. This excluded the first hypothesis, as starting with the experimental Phase II structure initially for calculations should not require overcoming the energy barrier during the geometry optimisation if it was indeed the DFT minimum. Another set of geometry optimisation calculations at 20.1 GPa was performed using again the experimental structure of Phase II as the initial, but this time allowing only for the atomic positions to relax, with unit cell dimensions constrained to their experimental values. Since the authors of [18] did not deposit the crystal structure of Phase II, Phase II was created by us using the experimental structure of Phase I and changing the values of the unit cell dimensions to those presented in [18]. This approach was deemed justified as it has been determined experimentally that Phase I and Phase II exist in the same space group: \(Pbca\), with \(Z^{\prime}=1\) and \(Z=8\). Additionally, since the conformational space of SA was very limited, we could assume that the same anti-periplanar conformation was present in both forms.