  In Fermionic Molecular Dynamics the occurrence of multifragmentation depends
strongly on the intrinsic structure of the many-body state. Slater determinants
with narrow single-particle states and a cluster substructure show
multifragmentation in heavy-ion collisions while those with broad wave
functions, which resemble more a shell-model picture, deexcite by particle
emission. Which of the two type of states occurs as the ground state minimum or
as a local minimum in the energy depends on the effective interaction. Both may
equally well reproduce binding energy and radii of nuclei. This ambiguity led
us to reinvestigate the derivation of the effective interaction from realistic
nucleon-nucleon potentials by means of a unitary correlation operator which is
much more suited for dynamical calculations than the G-matrix or the Jastrow
method. First results of mixing many Slater determinants are also presented.
