Illustration of the three steps required for running an externally irradiated shock model. The shock model shown here has a preshock density of 10⁴ cm⁻³, shock velocity of 20 km s⁻¹, and it is irradiated by a UV radiation field with G₀ of 10. The strength of the transverse magnetic field is 100 µG. First a chemical steady-state model is run, and the thermal, chemical, and excitation output is used as input for a PDR model. The output of the PDR model is then used as input for the shock model. The top row shows the temperature evolution across the model run, the middle row the abundances of H and H₂, while the bottom row shows normalized populations of the first five rotational levels of H₂. The “bump” in the temperature profile at t ~ 10² yr in the chemical steady-state model comes from reformation of a small fraction of H₂ on the grain, and the release of its binding energy.