Steady-state abundances of H₂ normalized to its value at Q∕Q_ref = 1 as a functionof Q∕Q_ref for two densities: n_H =10¹⁰ cm⁻³ (solid red line) and n_H = 10⁵ cm⁻³ (solid blue line). Other parameters are constant and equal to their fiducial values (see Table 2). Gas-phase formation of H₂ is dominatedby H⁻ for n_H = 10¹⁰  cm⁻³ (n_H∕W = 2 × 10¹⁶ cm⁻³) and by CH⁺ for n_H = 10⁵  cm⁻³ (n_H∕W = 2 × 10¹¹ cm⁻³). Dashed linesindicate H₂ abundance in the absence of dust. Abundances for each set of the model are normalized to their value at Q∕Q_ref = 1. We note that because of the low electron fraction at n_H = 10¹⁰ cm⁻³ (x_e ≃ 10⁻⁴), the formation on grains takes over from the formation via H⁻ (boundary ⑥) at a lower dust fraction than indicated in Fig. 3.