Steady-state abundances relative to total H nuclei for H₂ and chemical species involved in its formation for dust-free single point models with G₀ = 10⁴, W = 5 × 10⁻⁷, T_K = 1000 K, and n_H ranging from10⁵ to 2 × 10¹² cm⁻³. An analytical expression of the steady state abundance of H₂, assuming destruction by photodissociation and formation by H⁻ only, is alsoplotted in black dotted line (see Appendix C). Boundaries defined in Fig. 3 are also indicated on the upper axis. It is important to note that because of the decrease of the electron fraction following recombination at high density, a three-body reaction takes over from the formation via H⁻ (boundary ①) at a lower density than indicated in Fig. 3.