Flexible atom-pair tree. The left molecule represents the example molecule for the tree on the right side. The white 'R' marks the atom that serves as root atom (point of origin for the atom-pairs) for the tree. The black numbers symbolize the topological distance for the rigid atom-pairs. The red and green numbers correspond with the leaf numbers of the tree on the right side. The red or green color of these atom numbers indicates the membership of the atom to the rigid or flexible sub-tree, respectively.