  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.
