  Entrance-channel potentials in nucleus-nucleus collisions, relevant for the
synthesis of superheavy elements, are systematically studied within a
semi-microscopic approach, where microscopic nuclear densities of the colliding
spherical or deformed nuclei are used in semi-classical expressions of the
energy-density functional. From experimental data on fusion windows evidence is
found that the existence of pockets in the entrance-channel potentials is
crucial for fusion. Criteria for the choice of best collision systems for the
synthesis of superheavy elements are discussed.
