First-principles calculations were carried out using pseudopotential plane-wave DFT as implemented in the Vienna ab initio Simulation Package (VASP) code. Initial structures of the Pnma, R3 ̅ m, and P42/nmc phases of the bismuth chalcogenides were obtained from the Materials Project database and fully optimized using a plane-wave cutoff of 600 eV and the Γ-centred