In order to create realistic looking molecules, such as those in the ZINC data set, the mutations and choice of element are chosen with probabilities obtained by an analysis of a subset of the molecules in the ZINC dataset. An analysis of first 1000 molecules in the ZINC dataset shows that 63% of the atoms are ring atoms, so the the ring-creation or ring-insertion mutation is chosen 63% of the time. The most common 3-atom combination in rings is C=C-C, which accounts for 45% of all 3-atom combinations in rings (Table ), so a C∼C → C=C-C mutation is chosen 45% of the time, and similarly for the 34 other X∼Y∼Z combinations found in the data set. The site of insertion/creation is chosen randomly and excludes aromatic and six-membered rings. Similarly, for addition the most common bond involving at least one non-ring atom is C-C, so a C → C-C mutation is chosen more often (see Table for more details). A more specific bonding analysis, e.g. addition of C to C=C vs C-C, was considered but then the most probable bonding patterns are often not found in the early stages of molecule growth and the growth process effectively stops.