The calculated geometries for the various charge states adopted by V Se are shown in Fig. . In the neutral state (expected to be the dominant charge state under the typical weakly p-type Fermi levels reported in t-Se), the two dangling Se chains on either side of the vacancy remain separated and slightly contract the terminal Se-Se bonds to 2.25 Å (c.f. 2.36 Å for bulk t-Se). This results in a bipolaron state, where two hole polarons are localised to the Se p orbitals of the terminal atoms at the vacancy site, akin to the recombination-active Te/P-p bipolarons observed for V Cd 0* in CdTe 61 and V P -1 in NaP. The ground-state spin arrange- § Further in-depth analyses of the structural and electronic properties of point defects in t-Se are provided in Section S5. ¶ The e(+2/ + 1) defect level for V Se is located 5 meV above the VBM and so is not considered deep. ment for this bipolaron state is a singlet (S = 0), but the triplet (S = 1) state is calculated to be only 10 meV higher in energy. Using the ShakeNBreak 63,65 defect structure-searching method, an additional metastable geometry is also found for the neutral vacancy V Se 0* in which the two dangling Se chains ends displace toward each other to form a bond across the void, passivating their dangling bonds. This 'self-healing' type rearrangement is similar to that of iso-electronic, recombination-active V P +1 in the structurally-analogous NaP compound (further discussion in Section S5). However, unlike V P +1 in NaP, this geometry results in bond lengths and angles which deviate significantly from the ideal bulk values; 2.58 Å at the vacancy position and 2.44 Å for the two next-nearest Se-Se bonds in the chain, and bond angle deviations of up to 10 . This strain cost for V Se 0* , and the chargelocalisation cost for the bipolaron-type V Se 0 , result in significantly greater energy costs of formation for neutral vacancies compared to self-interstitials in t-Se. As shown in Fig. , the energy of this metastable state is predicted to be only 27 meV higher than the 'bipolaron' V Se 0 ground state, with a small 65 meV barrier for the structural transition, indicating that a non-negligible fraction of neutral Se vacancies will adopt this structure (⇠ 25 % at T = 300 K) -particularly during annealing and growth. The higher energy of the neutral Se vacancy, along with unoccupied single-particle states in the band gap, allows the emergence of charged vacancy species for various Fermi level positions in the band gap. For V Se +1 which is stable for Fermi levels close to the VBM, we see that one of the dangling Se chains bonds with a neighbouring chain, while for V Se -1 which is stable in the upper portion of the band gap, we retain a similar structure to V Se 0 with the single hole polaron now split across the two terminating Se atoms, as shown in Fig. . Lastly for V Se -2 which becomes stable for n-type Fermi levels near the CBM, the ground-state geometry involves the 'self-healed' vacancy structure as for V Se 0* , but with displacement of Se in a neighbouring chain toward the vacancy site.