In this work all local structure optimisations were performed using the GULP code . For the optimisation, all energies were calculated using pairwise interatomic potentials, with the Buckingham potential for the short-range interactions, with all potentials having a cut-off radius of 12 Å, unit cell parameters and atomic positions were optimised until the norm of gradient on forces was lower than 0.001(a.u.). Potential parameters for calculations with FUSE and MC-EMMA are shown in Tables and respectively. Table . Buckingham potential parameters used for calculations with FUSE, as used in previous work , all potentials use a cut-off distance of 12 Å.