H and H₂ abundances for a model clump of M = 10 M_⊙, n = 10⁵ cm^-3, and χ = 1000. The different lines denote different treatments of the H₂ formation. The solid line shows a model using the standard H₂ formation rate as given by Sternberg & Dalgarno (1989). The dashed and dotted lines show model results for H₂ formation according to CT where the formation on PAH surfaces is switched on and off, respectively. All three models assume a WD01-7 dust distribution. The green dash-dotted line assumes an MRN dust model and uses the CT H₂ formation.