  In the present study, water is considered as a dynamic network between
molecules at distances not exceeding 3.2 angstroms. The instantaneous
configurations obtained by using the molecular dynamics method have been
sequentially analyzed, the mutual orientation of each molecule with its
neighboring molecules has been studied and the interaction energy of each pair
of neighbor molecules has been calculated. The majority of mutual orientation
angles between molecules lie in the interval [10, 30] degrees. It has been
shown that more than 85% of the molecular pairs in each instantaneous
configuration form H-bonds and the H-bond network includes all water molecules
in the temperature range 233-293 K. The number of H-bonds fluctuates near the
mean value and increases with decreasing temperature, and the energy of the
vast majority of such bonds is much higher than the thermal energy. The
interaction energy of 80% of the H-bonding molecular pairs lies in the interval
[-7; -4] kcal/mol. The interaction energy of pairs that do not satisfy the
H-bond angle criterion lies in the interval [-5; 4] kcal/mol, and the number of
such bonds does not exceed 15% and decreases with decreasing temperature. For
the first time it was found that in each instantaneous configuration the H-bond
network contains built-in nanometric structural heterogeneities formed by
shorter H-bonds. The fraction of molecules involved in the structural
heterogeneities increases from 40% to 60% with a temperature decrease from 293
K to 233 K. These heterogeneities have a finite lifetime, but are constantly
present in the water. The number of large heterogeneities (containing more than
20 molecules) increases with decreasing temperature, and the number of small
structural heterogeneities (less than 20) decreases.
