Reference isobaric photochemical model for the Orion Bar with P = 10⁸ K cm^-3 and χ = 2 × 10⁴. We note the increase of n(H₂) as the gas temperatures decreases. The grey square shows the beam-averaged gas temperature estimated from our millimetre observations (T_k ≃ 150 K), which corresponds to A_V ≃ 1.5 in the model. The coloured squares show the beam-averaged abundances derived from the line survey.