Comparison of our simulation with existing observations of diffuse clouds, shown as grey dots (Crabtree et al. 2011; Indriolo 2012; Sonnentrucker et al. 2015). Top left panel: abundance of relative to H₂, top right panel: corresponding ζ_H2, bottom left panel: para-to-total ratio for both and H₂, and bottom right panel: H₂O relative abundance with respect to HF, where the HF column density in our simulation is extracted assuming a rigid scaling relative to H₂ (Indriolo 2012) with scaling factors in the range 0.5 × 10⁻⁸ − 3.5 × 10⁻⁸. The coloured 2D histogram represents the full distribution of pixels (0.25 pc wide) in our simulation, and the blue, orange, green, and red stars the average abundances from the identified filaments at different times. Finally, the magenta lines correspond to the theoretical abundance of assuming a nascent distribution (solid line), in which the formation of from cosmic-ray-induced ionisation of H₂ fully determines the relative abundance of the nuclear spin values, and a thermalised distribution (dashed line), in which the relative abundance is dominated by collisional exchange between and H₂ (Crabtree et al. 2011).