Variation predicted by different sequences of photoionization models of log η′_IR (left panels), log η′_AGN (middle panels), and log η′_AGN2 (right panels) as a function of the parameter α_OX (upper panels) and log U (lower panels). The models represented here were calculated with a stopping criterion when the fraction of free electrons is 98%. In all panels the solid lines represent models calculated with 12 + log(O/H) = 9.0, while dashed lines assume a value of 8.1.