Upper panels: abundances of selected non-deuterated (left), deuterated (middle), and grain-surface (both non-deuterated and deuterated, right) species as functions of time, as calculated with model T1 (solid lines) or model T2 (dashed lines). Middle panels: like the upper row, but comparing models T1 (solid lines) and T3 (dashed lines). Lower panels: like the other rows, but comparing models T1 (solid lines) and F1 (dashed lines). In all calculations, the density of the medium is set to n_H = 10⁵ cm^-3. The abundances of H₂O, NH₃, and NH₂D represent sums over their respective ortho and para states.