Observed and simulated NHD₂ spectra towards the column density peak of L1544. The red lines show simulations from the PH (top panel) and FS (bottom panel) reaction schemes. The same assumptions about the effective grain radius and the cosmic-ray ionisation rate have been used (a = 0.2 μm, ζ_H2 = 1.04 × 10⁻¹⁶ s⁻¹) in the two models. The simulation time has been chosen to provide an approximate agreement with the pNHD₂ spectrum.