For structural analysis (calculation of radial distribution functions and cation-4a displacements) we extracted a set of "inherent" structures from our simulation trajectory by performing a conjugate-gradient geometry optimisation on configurations selected every 50 timesteps. Each inherent structure represents a local minimum on the corresponding 3N-dimensional potential energy surface. To calculate an example electron localisation function (ELF) we performed full geometry optimisations with a cutoff of 500 eV with a minimum k-point spacing of 0.25 Å-1 , with atomic positions and cell parameters relaxed until all atomic forces were less than 2 × 10 -2 eV Å-1 . To obtain tin lone pair orientations, we calculated the set of maximally-localised Wannier functions for structures sampled every 50 ps, using the Wannier90 code . The net dipole on each tin atom is calculated by associating each Wannier center with the closest ion, and then, for each tin, summing over all associated Wannier-center displacement vectors . We assume that tin polarisation is dominated by contributions from the lone pair states, and that our calculated polarisation vectors therefore characterise these lone pair orientations.