Relative deviation of i_B upon Model (2) compared to Model (3) (see text for the description of the models) as a function of the H₂ volume density for selected temperatures and transitions. We note that HC₂NC and HNC₃ are represented in the left and right panels, respectively. These data were calculated for fixed column densities of 10¹² cm⁻². The grey band delimits the region where 10⁴ ≤ n (cm⁻³) ≤ 5 × 10⁴.