The predicted eRCN and DG spectra, presented as the absorption coefficient, α, in the 2.5 − 13.5   μm region as a function of E_g calculated using the structural de-composition described in Sects. 2.2.3, 2.2.4 and 2.3 and for the data in Table 2. The adopted colour-coding for this plot is from large band gap (2.67 eV, purple) to low band gap (–0.1 eV, black) with intermediate values in steps of 0.25 eV from 2.5 (violet) to 0 eV (grey) with the addition of the E_g = 0.1 and  − 0.1 eV cases (light grey and black, respectively), as per Paper II. The bands with central positions long-ward of the vertical grey line (λ(ν₀) > 7.3   μm) are not yet well-determined by laboratory measurements.