  A microscopic mean-field description of heavy ion collisions is performed in
the framework of the time dependent Hartree-Fock theory using a Skyrme energy
density functional. A good agreement with experiments is obtained on the
position of the fusion barriers for various total masses and mass asymmetries.
The excitation function of the 16O+208Pb is overestimated by about 16% above
the barrier. Transfer below the barrier is studied in 16O+208Pb central
collisions. Heavier systems are considered to study their fusion hindrance. We
also compute collision times of the 238U+238U system. The latter has been used
to produce super strong electric fields and to test non perturbative quantum
electrodynamics theory. Indeed, if the life time of such giant system is of the
order of few 10^-21 s, its electric field should induce spontaneous
electron-positron pair emissions from vacuum. In our calculations, highest
collision times are reached in the 238U+238U reaction for center of mass
energies between 1000 and 1300 MeV.
