Comparison between the population of J = 0 calculated using the approach described in Sect. 2 (n₀) against the population of J = 0 calculated using the methodology described in Sect. C. In the latter case, the coupling between the level populations in different regions of the cloud is explicitly taken into account by performing multiple iterations over the entire grid. The method described in Sect. 2 is accurate within 3% while at the same time the code is ∼10 times faster. In both cases, we use as input the physical model shown in Fig. 3 and use J ≤ 5.