Best-fit model for N₂D⁺ lines obtained with the fiducial model with initial cloud evolution and ζ₂ = 1.3 × 10⁻¹⁷ s⁻¹, at t = 10⁶ yr (red curves),overlaid to the observations (black). The used f_corr value is indicated at the top. The modelled lines produced using model with no cloud evolution at t = 10⁶ yr (and f_corr = 5) are shown in green. It is clear that the model with cloud evolution underestimates the (2–1) and (3–2) lines.