Sum of the residuals, over all available transitions, computed with models obtained by changing the f_corr value in the radiative transfer analysis. The synthetic spectra are produced using model at t = 10⁶ yr for all species but N₂D⁺, for which we use the best-fit time step t = 5 × 10⁵ yr. The dashed, blue line shows the quadratic function fitted to the R − f_corr relation. In each panel, the vertical black line shows the value that minimises the residuals, and the shaded grey area shows the corresponding 1σ interval. The f_corr value evaluated in Sect. 4 is shown by the vertical red line. The panels refer to N₂H⁺, N₂D⁺, HC¹⁸O⁺, and DCO⁺ (from left toright).