clear that the transition time for the SN atmosphere is much shorter than for the whole envelope. For instance, the atmosphere layers exceed $99\%$ and $99.9\%$ of their terminal velocities starting from nearly day 2.8 and day 11, respectively (Fig. \[fig:freexp\]).

It is interesting that a monotonic increase in the transition time with embedding inward the ejecta is broken at the $99.9\%$ level of the terminal velocity. There is a prominent feature at a velocity of $\sim$ 14500 kmc$^{-1}$ marked in Fig. \[fig:freexp\]. As we show later in Sect. \[sec:grg-lop\], it is the result of an additional acceleration due to the resonance scattering in numerous metal lines.

Homologous expansion is characterized by a density distribution frozen in velocity space and scaled in time as $\propto t^{-3}$. Such a density profile for the optimal model is shown at $t=50$ days in Fig. \[fig:denicl\]. In the inner region of the ejected envelope there is a dense shell with a density contrast of $\sim$ 20 produced by the $^{56}$Ni bubble at a velocity of $\sim$ 660 kmc$^{-1}$. The density distribution above the $^{56}$Ni bubble shell is nearly uniform up to $\sim$ 2000 kmc$^{-1}$. The outer layers with velocities in the ranges 3000–6900 km 