  The time-dependent transition between a diabatic interaction potential in the
entrance channel and an adiabatic potential during the fusion process is
investigated within the two-center shell model. A large hindrance is obtained
for the motion to smaller elongations of near symmetric dinuclear systems. The
comparison of the calculated energy thresholds for the complete fusion in
different relevant collective variables shows that the dinuclear system prefers
to evolve in the mass asymmetry coordinate by nucleon transfer to the compound
nucleus.
