Chemical evolution of H₂, H⁺, and H as a function of the total number density for Model III, which employs the equilibrium metal cooling at all temperatures (red curves), and Model IV, which employs the non-equlibrium metal cooling for temperature T< 10⁴ K (black curves and crosses). The effect of non-equilibrium cooling on the H₂ evolution is negligible and produces the same results as the equilibrium cooling model (crosses). A horizontal line at n_i/n_Htot = 0.5 is sketched to represent the fully molecular stage.