To demonstrate the effectiveness of SPPs in accelerating geometry optimisation, we selected three 4-element mixed-anion compositions that illustrate the optimisation problem for candidate solid Li ion conductors that we have previously studied: Li2Sn2S3Cl4, Li3Si3O5Cl5 and Li4BrOCl . Mixed anion solids have been getting attention recently as a promising materials design platform, however there is a limited number of force fields available for them because the bulk of the literature has focused on single anion systems. For example, such extensive datasets as openBV are missing some anion-anion pair potentials needed for each of these systems and therefore cannot be used off the shelf. To shortlist relevant ICSD structures that contain atomic environments relevant to the chemistry under consideration (i.e., to exclude structures containing short covalent bonds for O-O, Br-O, Br-S etc…), for each pair of atoms we defined a threshold value for the shortest allowed interatomic distance as listed in Supporting Information. Then the relevant ICSD structures were selected and used to create a set of proxy potentials for these three compositions.