The optical absorption spectrum calculated with the HSE06 functional in Figure shows a slow onset of absorption at 1.95 eV from the fundamental band gap, while the Tauc plot in Figure reveals a optical band gap of 2.28 eV. The irreducible representations analysis reveals the optical transition from the doubly-degenerate VBM to CBM is parity allowed but is expected to be weak. From the topmost valence band, the band gap is 1.95 eV all the way along the Γ-Z direction where the band edges are perfectly flat. In theory an optical transition could occur, but Z-Z transitions are parity forbidden. Transitions at both N and X are parity allowed, but as the curvature in both valence and conduction bands is greater in Γ-N and Γ-X directions, the band gap rapidly increases until other combinations of transitions around band edges are lower in energy. The VBM to CBM+1 (0.30 eV above CBM) transition is symmetry forbidden; however, VBM-1 (0.32 eV below the VBM) to CBM is allowed. The latter VBM-1 to CBM transition is stronger and with an energy gap of 2.26 eV lines up well with the Tauc band gap.