Predicted HCN and HNC abundances as a function of 1/T_K for different input parameters in our models. Data points and axes are similar to Fig. 4 b. From left to right and from top to bottom: model results for independent variations of the (a) energy barrier for reaction (2) (ΔE₂), (b) time evolution (τ), (c) gas density (n(H₂)), and (d) cosmic-ray ionization rate (ζ). The input values of these models are listed and colour-coded in each plot in the upper right corner of each subplot. Similarly, all fixed parameters for each of these comparisons are indicated in the lower left corner. These results are compared to our fiducial model assuming ΔE₁ = 200 K, ΔE₂ = 20 K, τ = 0.5 Myr, n(H₂) = 10⁴ cm⁻³, and ζ = 10⁻¹⁷ s⁻¹ (red solid line in all subplots).