corresponding hydrodynamic model. The obtained time-dependent structure of the atmosphere is then used to calculate synthetic spectra at selected epochs. The spectra are modelled by the Monte Carlo technique suggesting that the photosphere diffusively reflects the incident photons and that the line scattering is generally non-conservative and is described in terms of the line scattering albedo. The Thomson scattering on free electrons and Rayleigh scattering on neutral hydrogen are also taken into account.

Optimal hydrodynamic model {#sec:optmod}
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Elmhamdi et al. ([@edc03]) have constructed the “UBVRI” bolometric light curve of SN 1999em from the corresponding photometric data. To account for a possible contribution of the missing infrared bands, we add a value of 0.19 dex, taken from Elmhamdi et al. ([@edc03]), to the “UBVRI” luminosity and adopt the resultant light curve as the bolometric light curve of SN 1999em. Our aim is to find an adequate hydrodynamic model that reproduces the photometric and spectroscopic observations of SN 1999em. To fit the bolometric light curve of SN 1999em, various hydrodynamic models were explored. The bolometric light curve of SN 1999em is fitted by adjusting the pre-SN radius $R_0$, the ejecta mass $M_{env}$, and the explosion energy $E$, along with the density 