The calculated electronic structure of t-Se is shown in Fig. , exhibiting a slightly indirect fundamental band gap due to the splitting of the valence band maximum about the H ( 1 3 , 1 3 , 1 2 ) k-point. We calculate an indirect band gap of 1.71 eV and a direct gap of 1.83 eV, neglecting any electron-phonon coupling effects. As noted in the introduction, while some confusion has lingered in the experimental literature, the indirect nature of the t-Se band gap was observed in low-temperature optical measurements from the 1960s and 1970s -with DE g, direct/indirect ' 0.1 eV as predicted here. While the effect of SOC on the fundamental indirect band gap -and indeed for most electronic states across the Brillouin zone -is relatively small (DE g, SOC = 0.02 eV), the effect on the direct band gap is significant, decreasing it by DE g, SOC, direct = 0.14 eV. The primary origin of this difference in direct band gap is a large upshift in the highest energy valence bands near the H k-point, due to strong SOC splitting of the band crossing at this point (Fig. ). Se solar cells typically employ thin films in the range of a few hundred nm to maximise crystallinity and carrier collection -and so indirect phonon-assisted absorption is insignificant, translating into a slight reduction in achievable J sc and thus efficiency (Dh ⇠ 1.5%) for a ⇠ 1.85 eV gap absorber. * For instance, improved crystallisation quality and adhesion is obtained for t-Se when deposited on isomorphic t-Te. The calculated and measured natural electronic band alignment of t-Se relative to the vacuum level is shown in Fig. , alongside values of some related materials for comparison. We see that t-Se exhibits similar band edge positions to the halide perovskite methylammonium lead iodide (MAPbI 3 ), indicating that similar choices of carrier transport layers could be made. t-Se has deeper valence band maximum (VBM) than the emerging PV material Sb 2 Se 3 as expected due to the anti-bonding Sb s -Se p lone-pair interaction at the VBM in Sb 2 Se 3 , contrasted with the non-bonding / weak inter-chain bonding Se p interactions at the t-Se upper valence band. The combination of low carrier effective masses along the Se chain direction (k) § and high band degeneracies at both band edges for t-Se results in high-frequency dielectric constants of e •,? = 6.71, e •,k = 10.28, which are large for a semiconductor with a band gap just under 2 eV, given the typical e • µ 1 p E g relation. In contrast, the ionic contributions to the static dielectric constant are extremely small due to the covalent bonding, giving an overall dielectric constant of e ? = 7.34, e k = 11.22, matching well with the experimental values of e ? = 7.43, e k = 12.24. As expected given the low-dimensional connectivity (Fig. ), significant anisotropy is found in the physical properties of t-Se, as exemplified by the e •,k/? values above and m h,? /m h,k ' 4 for the hole conductivity masses, along with the absorption profiles shown in Fig. where light propagating along the Se chain direction shows greater absorption at energies just above the band gap.