Spectral montage of the Hα region for model s15p2 at 350 d with shock power increasing upward from zero (label NoPwr) to 7 × 10⁴⁰ erg s⁻¹ (label Pwr7e40). Some of these models are used for comparison with observations in Section 5. A broader range of shock powers covering up to 10⁴³ erg s⁻¹ is used in Dessart & Hillier (2022). The flux offset between models is modest in the optical and greatest in the UV range, as discussed in Dessart & Hillier (2022).