Comparison of the relative HCO⁺ level population for benchmark problem 2a (upper panels) and 2b (bottom panels) at J = 1 (left) and J = 4 (right) obtained with SKIRT (blue solid lines with dots) and MAGRITTE (yellow solid lines) and the reference results of van Zadelhoff et al. (2002; gray dashed lines). The level population is normalized by the total number density. The relative differences between the MAGRITTE and SKIRT results are shown in the lower subpanels.