Cl. To avoid spurious forces associated with Pulay stress, equilibrium volumes were calculated using a series of constant volume calculations and fitted to the Murnaghan equation of state . Geometry optimisations were deemed converged when all atomic forces were smaller than 1 × 10 -3 eV/ Å. Supercell calculations used a 2 × 2 × 2 Monkhorst-Pack grid for sampling k-space.