Another question one may ask is: Can quaternary systems contain semiconductors with improved properties with respect to the ternaries? As an instructive example, let us consider the case of KAg 2 PS 4 (mp-12532). In terms of composition, this compound only differs from Ag 3 PS 4 (mp-12459) by the replacement of one Ag atom per formula unit by a K atom. Although the structures of the two materials are different, they are 3D in both cases. The band gap is predicted to increase from Ag 3 PS 4 to KAg 2 PS 4 (1.0 to 1.2 eV at the PBE level, 1.8 to 2.1 eV with HSE correction). Even though effective masses often tend to increase with increasing band gaps, the direction-averaged hole effective mass of KAg 2 PS 4 actually decreases by 0.48 m 0 , while the electron effective mass remains approximately constant. The resulting effective masses in KAg 2 PS 4 (0.25 m 0 for electrons, 0.78 m 0 for holes) are typical of high-quality optoelectronic semiconductors. This interesting trend might be explained in very qualitative terms by the inductive effect, in which the presence of an electropositive cation (here, K + ) increases the covalency of the remaining bonds (here, Ag-S), and therefore the dispersion of the bands.