Predictions for the fractional oH₂D⁺, N₂H⁺, and DCO⁺ abundances as functions of the cosmic-ray ionisation rate, ${\zeta }_{{\text{H}}_2}$ (a) time, (b), and gas density (c) according to our chemistry model. The first two (a and b) represent the column density ratios N(mol)/N(H₂) through the centre of the core. In panels a and c, the simulation time is 5 × 10⁵ yr. In panels b and c, ${\zeta }_{{\text{H}}_2}={10}^{-17} {\text{s}}^{-1}$. The dashed lines in panels a and b show the fractional abundances estimated via the LTE method in Sect. 3. In panel c, higher densities are on the left to be consistent with Fig. 6.