Evolution of the N₂D⁺/N₂H⁺ ratio with the H₂ OPR values at the beginning of core contraction for the slow (right) and fast (left) models. The bottom row corresponds to the density profile with peak density of 10⁵ cm^-3 and the top row to the profile with peak density of 2  ×  10⁶ cm^-3 (from Fig. 2). Initial OPR values are listed in the top left image. The full lines (OPR = 3) also trace the reference results (from Fig. 5).