Iteration scheme for the calculation of a PoWR atmosphere model. The inner cycle without the ray-by-ray radiative transfer is typically applied for a few (typically 5–6) iterations before the Eddington factors must be renewed. If the criteria for a stratification update are fulfilled – see Sect. 5.4 for the case of a HD update – this is performed before the next ray-by-ray transfer would be scheduled. On the right side of the figure, indicated by gray arrows, the most important quantities obtained by the current step are highlighted.