The favoured phase of Se depends on temperature, pressure and synthesis procedures (e.g. epitaxial growth on lattice-matched substrates). To assess the relative stability of the various crystalline structures, we compute the athermal formation energies of each low-energy (DE hull  0.2 eV/atom) experimentally-observed Se allotrope on the Materials Project database, using hybrid DFT including dispersion corrections (HSE06+D3) -as shown in Table 1.