Illustration of the model grid from Sect. 5. The plot shows the effective temperature T_eff and the photospheric density ρ_ph for all atmosphere models. At low temperatures the actual photospheric density can exceed the targeted upper limit of 8 × 10⁻¹⁴ g cm⁻³, due the development of a strong recombination wave, which complicates the mapping between photospheric properties and the computational grid.