Sketch of our procedure to calculate synthetic spectra and light curves from the results of SN Ia explosion model calculations. M(⁵⁶Ni) is the total mass of ⁵⁶Ni and Z represents the chemical composition of the supernova ejecta, which in general has a radial stratification (cf. Sect. 2). In a first step the time-dependent radiative transfer is solved, determining the energy deposition rates E_dep(v,t) and the luminosity L_IB(t) at the inner boundary of the non-LTE model along with the light curve. is the emission coefficient resulting from the energy deposition rates (cf. Sect. 3); L_IB(t), the luminosity used for the inner boundary condition of the non-LTE model (cf. Sect. 4), is obtained from the solution of the time-dependent radiative transfer of the deeper layers. For the outer part of the ejecta a snapshot calculation for a stationary non-LTE model atmosphere is performed, using the energy deposition rates and the luminosity from the time-dependent radiative transfer. As a result this simulation yields the temperature structure, the occupation numbers along with the ionization structure, the radiation field, and the synthetic spectrum. The non-LTE model-part shows the non-linear system of integro-differential equations that form the basis of stationary atmospheric models, where the detailed statistical equilibrium determines the occupation numbers n_i of the atomic levels i,j via the collisional (C_ij) and radiative (R_ij) transition rates; the spherical radiative transfer equation yields the radiation field (χ_ν and S_ν are the total opacity and source function of all microphysical processes that are explicitly considered) expressed by the specific intensity I_ν, the mean intensity J_ν and the Eddington flux H_ν; and the microscopic energy equation of matter⁴ gives the temperature structure T(r) within the simulated part of the supernova ejecta (cf. Sect. 4). r is the radial coordinate, ρ the mass density, v the velocity, p the gas pressure, and e the internal energy; the left-hand side of the energy equation contains the change of the internal energy and the adiabatic cooling term, and the right-hand side is the negative of the 0th moment of the transfer equation describing the total radiation field.