Comparison between two identical clouds, with (top panel) and without (bottom panel) H₂ formation on coronene. H₂ formation rate coefficients on ionized (solid) and neutral (dashed-dotted) coronene and dust grains (dashed lines) are represented on the left y-axis. Number densities of H and H₂ (dotted lines) are represented on the right y-axis. The thick vertical green line indicates the location where n(H) = 2·n(H₂). G₀ = 10^3.5 and n_H = 10⁵ cm^-3.