On the Physical Cluster of Nucleation Theory: Generalized Hill Cluster
  The physical cluster originally conceived by Hill (J. Chem. Phys, 23, 617) is generalized for the case $N > 2$ in a novel way. Contrary to Hill's pairwise generalization, this definition assures that all constituent molecules of the cluster have insufficient kinetic energy to escape, and avoids the spurious implication of Hill's generalization that the molecular velocities are to be increasingly equal. 
Rasmus A. X. Persson
cond-mat.stat-mech