NSGA-II 21 makes use of a crowding distance to differentiate within the splitting front. The crowding distance is calculated for each individual, and indicates how closely the individual is surrounded by the other members of the splitting front. For NSGA-II, the crowding distance used is the Manhattan distance in objec-tive space. A larger crowding distance indicates a less crowded individual. Within a splitting front, NSGA-II orders all individuals by their crowding distances, and subsequently accepts the molecules with the largest crowding distance into the evolutionary population until the maximum size is reached. The outer solutions in the splitting front are assigned an infinite crowding distance to ensure that they are retained in each generation.