Abundance profiles for the four analysed species (from left to right: N₂H⁺, N₂D⁺, HC¹⁸O⁺, DCO⁺) at t = 10⁵ (top row), t = 5 × 10⁵ yr (middle row), and t = 5 × 10⁶ yr (bottom row). The colours refer to different models used for the CR ionisation rate: the fiducial model with ζ₂ = 1.3 × 10⁻¹⁷ s⁻¹ is shown in blue, whilst the red and green curves show the models and of PI18, respectively.