One-zone non-LTE calculation of $L_{\text{HCN}}^{\prime }/L_{{\text{HCO}}^{\text{+}}}^{\prime }$ as a function of H₂ volume density. Gas kinetic temperatures of 20, 50, and 100 K and H₂ column densities of 10²², 10²³, and 10²⁴ cm⁻² were considered. The line width was fixed to be 50 km s⁻¹ for all models. Different color curves represent various fractional abundances of HCN and HCO⁺ as indicated in the legend. Solid curves correspond to models without electron excitation, while dashed and dotted curves take into account excitation by electrons of different fractional abundances. Spikes seen in the lower right panels are caused by the population inversion of HCN. The gray shades indicate the range of 0.4−2.3, corresponding to the global $L_{\text{HCN}}^{\prime }/L_{{\text{HCO}}^{\text{+}}}^{\prime }$ found in the sample galaxies. See Sect. 6.1 for details.