Equilibrium concentrations at disc midplane shown for some representative components of the Umebayashi-Nakano (left panel) and the modified Oppenheimer-Dalgarno network (right panel), respectively. Apart from s_e = 0.3 the parameter setup is taken from Sano et al. (2000). The elements m and M of the modified Oppenheimer-Dalgarno network are specified by molecular hydrogen and magnesium. The line x_i = ∞ at R = 10   AU is marked to aid comparison with the modified Oppenheimer-Dalgarno network applied to dust agglomerates, cf. Figs. 4–6.