For the end member SrBi3O4Cl3 (x=1) structure in the SrxBi4-xO4Se1-xCl2+x series, a single unit cell structure was considered with 50 % occupancy of Sr and Bi on the Bi2 site. The same Ca/Bi ordering was used for the CaBi3O4Cl3 unit cell structure. For the Bi4O4SeCl2 (x=0) end member, and intermediate Sr0.25Bi3.75O4Se0.75Cl2.25 (x=0.25) and Sr0.75Bi3.25O4Se0.25Cl2.75 (x=0.75) systems, a √2 × √2 × 1 supercell expansion of the initial unit cell was used to explore different Se/Cl orderings. For the end member SrBi3O4Cl3, Bi4O4SeCl2 and CaBi3O4Cl3 structures, the lattice parameters and atomic positions were optimised until the force on any atom fell below 0.005 eV/Å, with and energy converged of 10 -7 eV. For intermediate compositions, x=0.25 and 0.75, the lattice parameters were initially fixed those of the lowest energy x=0 and x=1 phases, respectively, and the energies of different Se/Cl orderings were compared, after optimisation of the atomic positions with the same force tolerance of 0.005 eV/Å. The lattice parameters of the lowest energy x=0.25 and 0.75 structures predicted under fixed cell conditions were then fully optimised before calculation of the electronic structure.