DFT simulations. Ab initio DFT calculations were performed using the numeric atomic orbital (NAO) package, FHI-aims, which is an allelectron, full-potential electronic structure code. The generalised gradient approximation (GGA) of Perdew-Burke-Ernzerhof, reparametrized for solids (PBEsol), was used as the exchange-correlation (XC) functional during geometry optimisations, with the XC functional of Heyd, Scuseria and Ernzerhof (HSE06) and a screening parameter, w, of 0.11 bohr -1 used for subsequent single point energy calculations on the optimised structures. Dispersion interactions were accounted for using the Tkatchenko-Scheffler method, which is a pair-wise additive approach to include van der Waals interactions within the system. Calculations were performed using a 'light' basis set of the 2010 FHI-aims release, with SCF convergence assumed when the change in density was 10