For both substitutions and interstitials, chalcogens tend to be the lowest energy impurities (though are electrically inactive), while halogen/pnictogen impurities are highest in energy and H-related defects are intermediate. Further analysis of defect geometries and electronic structures of impurities in t-Se is given in Section S5. We note that Refs. 37,93 also performed initial screenings of some point defects in t-Se using semi-local DFT, however, we obtain qualitatively different results for most defect properties. We attribute these discrepancies to the use of semi-local DFT functionals, strained supercells, and neglect of vdW interactions and structure-searching (Section S5.5) , giving shallower defect levels, unphysically low capture coefficients, and negative formation energies.