Fig. 2. Examples of the four transformation operations implemented in the evolutionary algorithm. (a) The addition operation, illustrated with three possibilities for the addition of a pyridine fragment to a pyridine molecule, forming a naphthyridine type molecule; (b) crossover between two aza-napthalene molecules showing one crossover possibility for the example parent molecules. Additional possibilities can occur due to freedoms in the orientation of the fragments when combined together and the possible pairings of each fragment. (c) The recombination transformation of an aza-anthracene molecule, creating an isomer of the initial molecule. Additional possibilities can occur due to freedom in the fragmentation positions, fragmentation position moves and the orientation of the fragments when combined together. (d) The mutation transformation on the pyridine molecule with a nitrogen atom fragment, showing three possible mutations forming either a pyridazine, pyrimidine or pyrazine molecule.