Results from the validation retrieval on the 3 – 20 μm, R = 1000, and photon noise S/N = 50 Venus-twin spectrum (opaque H₂SO₄ clouds) using the opaque H₂SO₄ cloud forward model (see Sect. A). Lower left half : Corner plot of the posterior distribution of the forward model parameters. Here, L(·) stands for log₁₀(·). We derived the equilibrium temperature T_eq and the Bond albedo A_B from the other posteriors (see Appendix B). The dotted black lines indicate the true values. In the 1D posteriors, we show the 16th, 50th, and 84th percentiles as black dashed lines. Top center: P–T profiles corresponding to the retrieved P–T parameters. Color-shaded regions indicate percentiles of the retrieved P–T profiles. The gray shaded regions indicate percentiles of the retrieved cloud-top pressure. The solid black line, the orange circular marker, and the red square marker represent the true Venus-twin P–T profile. In the inset figure, we plot the 2D P₀-T₀ posterior. Top right: A table summarizing the true values of the forward model parameters and the 16th, 50th, and 84th percentiles of the parameter posteriors. Center right: Comparison of the retrievals best fit to the Venus-twin input spectrum. The thin colored line represents the best fit, the thick black line the Venus-twin input spectrum. The uncertainties on the best fit are too small to be visible. Below the spectrum plot, we show the mean wavelength-dependent contribution of the atmospheric layers to the emission spectrum corresponding to the retrieved parameter posteriors. Darker colors indicate higher contributions. The dashed black line indicates the position of the cloud-deck assumed to simulate the input spectrum (opaque H₂SO₄ clouds).