  Chemical substitution, combined with strain, allows the charge-transfer
energy in hole-doped cuprates to be broadly tuned. We theoretically
characterize the structural and electronic properties of the family of
compounds $R_2$CuO$_2$S$_2$, constructed by sulfur replacement of the apical
oxygens and rare earth substitutions in the parent cuprate La$_2$CuO$_4$.
Additionally, the enthalpies of formation for possible synthesis pathways are
determined.
