Comparison of the chemical network in HYACINTH with NL99 and G17 networks. The abundances of H₂, CO, C, and C⁺ as a function of the visual extinction A_V in a semi-infinite plane-parallel slab are shown in different panels. The left column shows the abundances for a CRIR (ζ_H) of 10^–17 s^–1 H^–1 while the right one has ζ_H = 2 × 10^–16 s^–1 H^–1. The blue, turquoise, and red lines represent the results from HYACINTH, extended HYACINTH, and NL99 respectively. Here extended HYACINTH refers to the HYACINTH network with additional chemical reactions for He⁺ and HCO⁺ that are not part of standard HYACINTH (see text for more details). The dashed and dotted black lines show, respectively, the abundances from the chemical network and the PDR code in G17. The slab has a uniform hydrogen density n_II = 1000 cm^–3, solar metallicity and solar dust abundance, and is illuminated from one side by a UV field of strength χ = 1.