CLUSTER_NATOMS
This is part of the clusters module
It is only available if you configure PLUMED with ./configure –enable-modules=clusters . Furthermore, this feature is still being developed so take care when using it and report any problems on the mailing list.

Calculate the number of atoms in the cluster of interest

Examples
Glossary of keywords and components
Description of components
Quantity Description
.#!value the number of atoms in the cluster
Compulsory keywords
CLUSTERS the label of the action that does the clustering
CLUSTER ( default=1 ) which cluster would you like to look at 1 is the largest cluster, 2 is the second largest, 3 is the the third largest and so on.