  In equilibrium and supercooled liquids, polymorphism is manifested by
thermodynamic regions defined in the phase diagram, which are predominantly of
different short- and medium-range order (local structure). It is found that on
the phase diagram of the water model, the thermodynamic region corresponding to
the equilibrium liquid phase is divided by a line of the smooth liquid-liquid
crossover. In the case of the water model, this crossover is revealed by
various local order parameters and corresponds to pressures of the order of
$3\,150 \pm 350$ atm at ambient temperature. In the vicinity of the crossover,
the dynamics of water molecules change significantly, which is reflected, in
particular, in the fact that the self-diffusion coefficient reaches its maximum
values. In addition, changes in the structure also manifest themselves in
changes in the kinetics of hydrogen bonding, which is captured by values of
such the quantities as the average lifetime of hydrogen bonding, the average
lifetimes of different local coordination numbers, and the frequencies of
changes in different local coordination numbers. An interpretation of the
hydrogen bond kinetics in terms of the free energy landscape concept in the
space of possible coordination numbers is proposed.
