Left panel: comparison of the [O I] λλ6300, 6364 line strength for three different rates for C + O⁺ → C⁺ + O (¹D). The simulated model is the he4p0 model from Barmentloo & Jerkstrand (2025), evaluated at 300 days after explosion. Line strengths decrease by about 10% compared to the model with the rate used in Jerkstrand et al. (2011). Right panel: same models as in the left panel, but now comparing the strengths of [C I] λλ9824, 9850. Note that the high Doppler blending makes the doublets appear as single, broad features.