1Such triplets are relatively efficient, since they allow us to deal with clusters that exhibit a small variation in nearest-neighbor atom distances: since T would be quite sensitive to such differences, possible deviations would propagate when extending the map to the full cluster and thus make it impossible to match the farther-out atoms under the mapping T.

2In order to increase the efficiency and robustness of the algorithm, one should choose as targets atoms of a type of which only a few atoms are present in structure B. If no reference atom type is explicitly selected by the user, CCL employs the atom type with the least number of atoms present in B as a default reference type. Note that in principle it is also possible to assign the same working name to all atoms involved, a situation possibly useful for e.g. solid solution alloys.

3The ten atoms occupy entries 3-12 in the atom list; atoms 1 and 2 in the list are dummy atoms that serve only as reference points.

4KPLOT (Hundt, 2011BB8) is a standalone program, and can be obtained for free from R. Hundt ( or freely downloaded from the Crystal Impact web site ( ).