These hydrogen atoms were put on the mirror plane perpendicular to a on which the O18 and O19 atoms also reside (Wyckoff position 96i). The structure was then converted to the primitive cell prior to full DFT optimisations using the CP2K code, version 2023.1. As described in the main text, separate optimisations were performed using the PBE-D3 and rev-vdW-DF2 functionals. The DFT calculations used "molecularly optimised" double-zeta basis sets (DZVP-