Static DFT calculations were performed with the VASP package 22 using the Perdew-Burke-Ernzerhof (PBE) functional and the Projector Augmented Wave (PAW) pseudo-potentials distributed with VASP 5.4.1. Dispersion corrections were included using the Grimme DFT-D3 method with Beck-Johnson damping. Full details of the settings used are provided in the supplementary information (SI Section 2). For full geometry optimisations of the unit-cell and the molecular geometries, the default optimisers within VASP were used. For those calculations which required constraints the GADGET optimiser was used, which has an interface to the VASP package.