Natural clustering: the modularity approach
  We show that modularity, a quantity introduced in the study of networked systems, can be generalized and used in the clustering problem as an indicator for the quality of the solution. The introduction of this measure arises very naturally in the case of clustering algorithms that are rooted in Statistical Mechanics and use the analogy with a physical system. 
L. Angelini, D. Marinazzo, M. Pellicoro, S. Stramaglia