Mass-weighted 1: 1 comparison plots between analytical calibrators and numerical results for the abundance ratio, χ(HCO⁺)/χ(CO). The CR model is annotated in each panel. The dashed black line represents the 1: 1 reference, and the dashed green line shows the linearity of the data and the relative deviation in linearity among the models. The data are filtered under the constraints UV <0.1 G₀ and n_H>10³ cm⁻³ for clarity.