Analysis of the changes in unit cell volume and molecular geometry as a result of systematically changing the pressure from 0 to 20 GPa (see SI for details), showed that, apart from Form I, there was a smooth change in the geometric parameters over the full pressure range. Form I, however, showed discontinuities in several geometric parameters but especially the C-S-H bond angle (Figure ). This increased from an initial value around 96.8 • to 99.2 • at around 7 GPa when it drops to below 96 • . This structural change may be consistent with that observed between 6.9 and 7.8 GPa by a Raman study of Form I, where it was attributed to a possible breaking of the sulfhydryl hydrogen bonds. These results are also consistent with the Raman study of Forms I and II, which indicated that Form II behaved reversibly under pressure, whilst Form I did not; indicating substantial irreversible conformational changes in Form I as the pressure is altered. A sudden change in angle also occurs at 16 GPa for both Forms I and III.