  Quantum Molecular Dynamics (QMD) calculations of central collisions between
heavy nuclei are used to study fragment production and the creation of
collective flow. It is shown that the final phase space distributions are
compatible with the expectations from a thermally equilibrated source, which in
addition exhibits a collective transverse expansion. However, the microscopic
analyses of the transient states in the intermediate reaction stages show that
the event shapes are more complex and that equilibrium is reached only in very
special cases but not in event samples which cover a wide range of impact
parameters as it is the case in experiments. The basic features of a new
molecular dynamics model (UQMD) for heavy ion collisions from the Fermi energy
regime up to the highest presently available energies are outlined.
