## 1 Introduction
Given its wide-ranging applications, SA is indispensable for use in harsh environments. The crystal structure of SA has been thoroughly analysed under ambient conditions. SA crystallises in the orthorhombic space group P_ba_ with Z' = 1 and Z = 8. In the ICSD database, four similar structures of SA can be found, with the most recently published ones obtained from X-ray diffraction and characterised by the following cell parameters: a = 8.034(1) A, b = 8.020(1) A, c = 9.236(2) A; and neutron diffraction with the following cell parameters: a = 8.036(3) A, b = 8.025(3) A, c = 9.236(5) A [17]. Despite the small size of SA molecules, the hydrogen bonding arrangement in its crystal lattice is quite complex. In the solid state, SA exists in the form of zwitterions, characterised by anti-perilplanar conformation. Both ends of this molecule participate in independent hydrogen bonding with neighbouring molecules; the NH3+ group is surrounded by five oxygen atoms at short interatomic distances (about 2.79-2.95 A), but only three of those oxygens are directly implicated in hydrogen bonding. It should be also noted that the shortest N-O distance is not in the direction of any hydrogen atom. The zwitterionic form of SA, which boosts the intermolecular electrostatic interactions, and presence of multiple strong H-bonds, results in distinctive physical features of this compound, including a high melting point of around 205 \({}^{\circ}\)C, unlike other sulfonic acids (Figure 1). Recently, it has been discovered experimentally that SA undergoes pressure-induced IPT, resulting in the abrupt decrease in \"c\" and increase in \"a\" and \"b\" unit cell lengths (Figure 2) [18]. The authors of [18] have used Raman spectroscopy to additionally characterise this transition of initial Phase I into newly discovered Phase II, starting at approximately 10.0 GPa. Although the authors of [18] have performed the Rietveld refinement of the powder pattern of Phase II recorded at 13.7 GPa, they have not published nor deposited in a database the final coordinates after refinement, presenting the structure of Phase II solely in a form of a Figure (Figure 9 in [18]) and the unit cell dimensions resulting from the analysis of the PXRD patterns. As we have shown in a recent review [19], computational methods developed to mimic the solid state are typically calibrated using structural data at an ambient pressure, making their application to high-pressure scenarios a significant challenge for validating  
Figure 1: Unit cell of SA in a perspective projection. Blue dashed lines represent the hydrogen bonds. Figure 2: Compression of (**a**) lattice constants and (**b**) unit cell volume of SA with respect to pressure. Shadow represents the phase transition region. Reprinted from [18], with the permission of AIP Publishing. their computational accuracy. Nonetheless, quantum chemical calculations, particularly utilizing the density functional theory (DFT), have consistently demonstrated their efficacy as a valuable instrument for predicting properties that can subsequently be validated by experimentalists, as well as elucidating, at the molecular level, the findings of high-pressure experiments. On the one hand, the application of high pressure can result in the formation of the new phase, which makes this approach a tempting direction for widespread exploration and application. On the other hand, high-pressure experiments are experimentally demanding; therefore, the methods that can be accurately used to guide those experiments and predict their outcomes are highly anticipated. Therefore, the aim of this work was to assess if the periodic DFT methods can predict the pressure-induced polymorphic phase transition of SA, describe the thermodynamic aspects of this transformation and explain the differences in the Raman spectra observed experimentally.