Left: χ² map for the grid run of constant density and constant temperature with Cloudy. The red star represents the model with the lowest χ² of 1.94 (see text). The red crosses mark the grid values where the Cloudy code raised a failure flag of failed assert or cloudy abort. Right: Ro-vibrational diagram of H₂ comparing our observations (markers) with the best Cloudy model (lines) for T = 600 K and log₁₀(n_H /cm⁻³) = 3.9. Both models and observations have been normalised to the column density over the statistical weight of the 1–0 S(1) line at 2.12 µm (i.e. N_r/g_r). Markers and lines are colour-coded by vibrational level (v), and circles and solid lines represent ortho- transitions, whereas triangles and dashed lines are for para-transitions.