Solid solution cluster expansions (CE) are fitted for both systems using the ICET package . For the Na 1 -x K x Cl solid solutions, cutoff of 15.0 Å and 8.0 Å are used for two-and three-body clusters, respectively, as determined from cross validation (See Fig. -S3 in the SI). For Ag 1 -x Pd x 13.0 Å, 7.0 Å and 6.0 Å cutoffs are used for two-, three-and four-body clusters as done by Ångqvist et al. . The cluster expansions are fitted with automatic relevance detection regression (ARDR) as implemented in ICET.