Observations compared to TDR model predictions. Data (open circles): column densities of HCO⁺ and OH from Lucas & Liszt (1996; top left panel), H₂O from Flagey et al. (2013; top right panel), C₂H from Lucas & Liszt (2000; bottom left panel), and the excited levels of H₂ from Gry et al. (2002) and Lacour et al. (2005; bottom right panel). For clarity, in the bottom right panel the points for a given level are slightly shifted on the abscissa. Models (filled symbols): computed for A_V = 0.2 (red), 0.4 (magenta), and 0.6 (blue) mag and n_H = 20 (crosses), 30 (triangles), 50 (squares), 100 (circles), and 300 (upside-down triangles) cm^-3. Except for the bottom right panel, a increases along each curve from 10^-11 to 10^-10 s^-1 from right to left. In the bottom right panel, the models are shown for A_V = 0.1 mag (red) and 0.4 mag (blue) and single values of the rate-of-strain a = 3 × 10^-11 s^-1 and the density n_H = 100 cm^-3.