The real and imaginary part of the optical potential between two heavy ions is discussed and calculated starting from a realistic nucleon-nucleon interaction. Starting from the collision of two infinite nuclear matters flowing through each other the real and the imaginary part of the heavy ion nuclear potential is calculated for the energies up to 100 MeV per nucleon in the laboratory frame. The input is a realistic nucleon-nucleon interaction. The solution of the Bethe-Goldstone equation for the collision of two infinite nuclear matters yields a complex reaction matrix. The transition to two finite nuclei is done by a generalized local density approximation. The contribution from collective surface vibration can not be included in nuclear matter. They are taken into account either by eliminating them according to the Feshbach prescription or by including them explicitly by a coupled channel description. Good agreement for differential and total cross sections are obtained. Remaining discrepancies at smaller energies are attributed to particle transfer not included in the description.