Pseudopotential plane-wave density-functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) code. Initial crystal structures of Pnma GeS and GeSe were taken from Ref. , and the R3m structures of the two chalcogenides were derived from the GeTe structure reported in Ref. by replacing the chalcogen atom. The rocksalt structure of GeSe was taken from Ref. , and rocksalt GeS was prepared from this by substituting Se with S.