[en] The potential barriers and the nuclear shapes are calculated in the fragmentation theory for the various target-projectile combinations. It is shown that the choice of the reaction partners in a compound nucleus reaction can be optimized on the basis of the interaction barriers and associated nuclear shapes. The mass and charge asymmetries of the reaction are shown to be related with the interaction length, which is defined as the length of nucleus as the top of the interaction barrier. Combinations with largest mass asymmetries and interaction lengths are shown to be most favourable for the production of super-heavy elements in compound nucleus reactions. The calculations are illustrated for the compound nuclei ²⁵⁸104 and ²⁶⁰106. (author)