Abundances of selected species as functions of time at different densities (labeled in the figure), in a gas-phase model at T = 10 K, using different approaches to H₂ formation. Dash-dotted lines correspond to the gas-phase reactions of Kong et al. (2013). Solid lines correspond to grain-surface H₂ formation assuming only H and D adsorption and with the binding energies of H and H₂ and their deuterated isotopologs set to 100 K. Dotted lines (superimposed on the dash-dotted lines) correspond to grain-surface H₂ formation assuming only H and D adsorption and with the binding energies adopted in the main body of this work. The abundances of ammonia, water and H₂D⁺ represent sums over the abundances of their respective ortho and para states.