The Li 2 FeSO structure found by the search here (Fig. , space group Pc) contains Li and Fe atoms with substantial displacements from their sites in the cubic disordered anti-perovskite cell (Fig. , space group Pm 3m). This result can be easily rationalised by invoking our knowledge of perovskite tilting, as the O, Li, and Fe atoms have sizes such that the OLi 4 Fe 2 network contains tilted octahedra in the a 0 b 0 c + pattern using the Glazer's rotation , which is consistent the Goldschmidt tolerance factor of 0.85 computed from the ionic radii. This value is computed using weighted average of Li and Fe radii and the ionic radius of S 2-in six-fold coordination. The latter may result in underestimation of the tolerance factor as S 2-is in fact under an 12-fold coordination in the anti-perovskite structure. The previously reported theoretical ground state structure of Li 2 FeSO, which obtained by enumerating Li and Fe sites (shown in Fig. , space group P4 2 mmc), does not have any octahedral tilting , and it is 6 meV higher in energy compared to our structure obtained using AIRSS. This high-symmetry structure is dynamically unstable, and following the imaginary mode at the M point results in a -b -c 0 tilted octahedra (space group P2/c). The final energy of this distorted structure is almost identical to the one found with AIRSS. Oxysulphides containing Fe 3+ are mostly unstable and far from the hull, and all of the phases in Table contains Fe 2+ instead. This may be attributed to Fe 2 O 3 being very stable as one of the competing phases.