Spectral synthesis model of the optical and NIR nebular spectrum. The optical spectrum was obtained on +229 days and the NIR spectrum on +192 days (Hoeflich et al. 2021). Over-plotted in blue is the synthetic spectrum computed from a M_Ch C + O WD disrupted following an off-centered delayed detonation explosion. This model is computed with a viewing angle of −90° as defined by the schematic in Fig. 19. At the blue end of the optical spectrum the model underestimates the flux of several Fe II+ Fe III complexes. After recomputing the explosion model with the addition of a 2000 km s⁻¹ macroscopic mixing scale of the ⁵⁶Ni-Fe into the lower density Ca-layer (see Fig. B.1), we obtain the synthetic spectrum shown in red. The mixing model better reproduces the [Fe II], [Fe III], [Co II], and [Co III] line complexes between ∼4000 − 6000 Å and ∼8500 − 9000 Å. The observed spectra have been scaled arbitrarily to match the model results.