  We employ the Distance Matrix method to investigate metabasin dynamics in
supercooled water. We find that the motion of the system consists in the
exploration of a finite region of configuration space (enclosing several
distinct local minima), named metabasin, followed by a sharp crossing to a
different metabasin. The characteristic time between metabasin transitions is
comparable to the structural relaxation time, suggesting that these transitions
are relevant for the long time dynamics. The crossing between metabasins is
accompanied by very rapid diffusional jumps of several groups of dynamically
correlated particles. These particles form relatively compact clusters and act
as cooperative relaxing units responsible for the density relaxation. We find
that these mobile particles are often characterized by an average coordination
larger than four, i.e. are located in regions where the tetrahedral hydrogen
bond network is distorted.
