The dispersion-corrected PBEsol+D3 method was chosen to model the electron exchange and correlation based on our previous work. Projector-augmented wave (PAW) pseudopotentials were used to model the ion cores with the following valence configurations: Ge 3d 10 4s 2 4p 2 ; S 3s 2 3p 4 and Se 4s 2 4p 4 .