Comparison of the variations of the NLL as a function of the volume density of the inner layer for the LoS A. We used a sandwich model where N(¹³CO) and N (C¹⁸ O) were fitted, while the column densities of ¹²CO, HCO⁺, and H¹³CO⁺ were computed assuming fixed abundances relative to ¹³CO. The top and bottom rows show the NLL variations for two values of N(HCO⁺)/N(¹³CO) varying by a factor of 2.0. The left panel shows the NLL variations for the ¹²CO and ¹³CO lines, while the right panel show that of the C¹⁸O, HCO⁺, and H¹³CO⁺ lines. In both panels, the black plain lines shows the variations of the NLL when fitting all the lines. The top row shows two local minima at similar NLL values while the bottom row only shows one local minimum. The vertical and horizontal black dotted lines intersect at the lowest NLL value, indicated by the red circle, and at the second local minimum that is indicated by the black circle, when it exists.