  A refined model for the unusual Type Ic supernova 1998bw, discovered as the
optical counterpart of GRB980425, is presented, and synthetic light curves and
spectra are compared with the observations. The first 30 days of the light
curve and the broad line features of the spectra can be reproduced with the
hydrodynamical model of the explosion of a 14$M_\odot$ C+O star, the core of a
star with initial mass 40$M_\odot$, assuming that the explosion was very
energetic (kinetic energy $E_{\rm K} = 5 \times 10^{52}$ erg) and that
0.4$M_\odot$ of $^{56}$Ni were synthesized. At late times, however, the
observed light curve tail declines more slowly than this energetic model, and
is in better agreement with a less energetic ($E_{\rm K} = 7 \times 10^{51}$
erg) one. This shift to a less energetic model may imply that the inner part of
the ejecta has higher density and lower velocities than the spherically
symmetric model with $E_{\rm K} = 5 \times 10^{52}$ erg, so that $\gamma$-rays
deposit more efficiently. We suggest that an aspherical explosion can produce
such a structure of the ejecta. We also study detailed nucleosynthesis
calculations for hyper-energetic supernova explosions and compare the yields
with those of normal supernovae.
