Fig. 2: Sn–Br phase diagram constructed using different mixing strategies for PBE(+U) and r2SCAN calculations.

a PBE(+U) only; b r2SCAN only; c naive mixing of formation energies, where all SnBr2 polymorphs are computed in r2SCAN and all other materials are in PBE; d the same set of energies as c, but employing our mixing scheme (Rule #2); e r2SCAN for all reference states (i.e., PBE(+U) ground states) and PBE(+U) for all other materials; f r2SCAN for all materials within 20 meV per atom of the PBE(+U) convex hull. The numerical value in parentheses indicates the energy above hull of the experimental ground state Pnma polymorph of SnBr2. The shaded blue regions represent the estimated range of experimental formation energies for SnBr237. Tabulated r2SCAN and PBE(+U) energies for all materials are available as Supplementary Data (see Supplementary Note 4).