Expected fractions of different species as a function of gas density at z = 0.2 for the HM12 (a harder spectrum; open symbols with dashed–dotted curves) and HM05 (a softer spectrum; filled symbols with solid curves) ionizing spectra in Figure 1. The calculations are performed using the Cloudy software. The fraction of H⁰ is found uniformly higher under HM12 than HM05 across the entire gas density range studied (f). At the same time, the fractions of ﹩{{\rm{C}}}^{2+}﹩ and Si²⁺ are lower at ﹩n({\rm{H}})\gtrsim {10}^{-3}\,{\mathrm{cm}}^{-3}﹩ under HM12 ((b) and (d)), while a reversed trend is seen under HM12 for C⁺ and Si⁺ at low densities, ﹩n({\rm{H}})\lesssim {10}^{-2}\,{\mathrm{cm}}^{-3}﹩ ((c) and (e)) and Mg⁺ displays very small changes (a) over the same gas density range. The differential corrections between hydrogen and other ions for different ionizing spectra naturally result in dramatically different estimates of gas metallicity for the same absorption systems (cf. Werk et al. 2014; Prochaska et al. 2017).