Comparison between emergent spectra calculated by our MC code and by Tardis. Upper panels: spectra comparison, Lower panels: relative difference between the spectral fluxes. Left: Model with only resonance scattering included. Every time the packet interacts with an atom, it is directly emitted in the random direction with the same CMF frequency. Right: Downbranch mode: resonance scattering and fluorescence allowed. The deexcitation is possible also to an arbitrary lower atomic level.