Same as Fig. A.1, but for a comparison between the classical and GP-aided approaches on the JWST/MIRI-LRS spectrum using ATMO2020++ (in green) and ATMO2020++ (no PH₃, in blue). Top panel : main molecular absorption features calculated using petitRADTRANS. In the other panels, the black lines show the observed spectrum; the colored lines show the corresponding model spectrum. The shaded regions around the model indicate the 1σ, 2σ, and 3σ dispersions from 200 random draws from the retrieved covariance matrix C. In the classical approach, the covariance matrix is assumed to be diagonal, C = ŝ² diag(σ²), where ŝ² is the noise scaling factor and σ the observational uncertainties. Zoom-in on the 5-11 μm region.