A plane wave energy cutoff of 300 eV and k-point density of 0.42 Å 1 (4 ⇥ 4 ⇥ 4 for the 3-atom P3 1 21 unit cell) were found to give total energies converged to within 1 meV/atom -using vaspup2.0, 52 and so were used in all calculations unless otherwise specified. A dense G-centred k-point mesh of 16 ⇥ 16 ⇥ 16, using tetrahedron smearing and downsampling of the Fock exchange matrix by a factor of 2 (to reduce computational cost with no loss to accuracy confirmed), was employed for bulk electronic structure and optical calculations to ensure a well-converged density of states (DOS). A convergence criterion of 0.01 eV/Å was imposed on the forces on each atom during geometry optimisations, and an increased energy cutoff of 550 eV was used to avoid Pulay stress. Charge carrier effective masses were obtained from non-parabolic fitting of the electronic band edges using the effmass , electronic band structure diagrams were generated using sumo and Galore 55 was used for photo-electron crosssections (Fig. ). Surface slab calculations to obtain vacuum band alignments were performed using surfaxe , for which slab and vacuum thicknesses of 20 Å each were found to give wellconverged electrostatic potential plateaus, for a surface cut along the (100) plane (i.e. between Se chains). Phonon calculations were performed using phonopy and ThermoParser 58 with the HSE06+D3 functional and a 4 ⇥ 4 ⇥ 4 supercell of the primitive t-Se unit cell.