Exploration of Case 1 fitting of the whole IRDC clump sample, including trends with density. Left column panels show fD versus n_H; middle column panels show [HCO+]/[CO] versus n_H; right column panels show [N2H+]/[CO] versus n_H. In all panels, the black points are the observed data, while grey points assume CO envelope contamination of 90% (see text). (a) First row: temperature comparison; models have ζ = 2.2 × 10⁻¹⁷ s⁻¹, A_v = 20 mag. Solid lines are from the models with T = 15 K and dotted lines from those of T = 16 K. (b) Second row: CRID comparison at 15 K; models have ζ = 2.2 × 10⁻¹⁷ s⁻¹, A_v = 20 mag and T = 15 K. Solid lines are from the models without CRID; dotted lines are with CRID. (c) Third row: CRID comparison at 16 K; results as (b), but with T = 16 K.