  In a freely cooling granular material fluctuations in density and temperature
cause position dependent energy loss. Due to strong local dissipation, pressure
and energy drop rapidly and material moves from `hot' to `cold' regions,
leading to even stronger dissipation and thus causing the density instability.
The assumption of `molecular chaos' is valid only in the homogeneous cooling
regime. As soon as the density instability occurs, the impact parameter is not
longer uniformly distributed. The pair-correlation and the structure functions
show that the molecular chaos assumption --- together with reasonable excluded
volume modeling --- is important for short distances and irrelevant on large
length scales.
  In this study, the probability distribution of the collision frequency is
examined for pipe flow and for freely cooling granular materials as well.
Uncorrelated events lead to a Poisson distribution for the collision
frequencies. In contrast, the fingerprint of the cooperative phenomena
discussed here is a power-law decay of the probability for many collisions per
unit time.
  Keywords: discrete element method, event driven simulations, clustering
instability, arching, shock waves, power-law distribution, cooperative
phenomena.
