Orientational relaxation dynamics of liquid water studied by molecular dynamics simulation

Orientational relaxation dynamics of water molecules in the liquid state are studied by molecular dynamics with TIP4P model. The biexponential decay of the dipolar autocorrelation function is associated with a heterogeneous distribution of local hydrogen bond patterns. The H-bond pattern was analyzed with a Voronoi polyhedra (VP) construction of the oxygen atom distributions. An asphericity parameter for VP was used to characterize the heterogeneous distribution of the local H-bond patches. The slow relaxation in the ordered region is temperature sensitive. It is associated with locally cooperative rotation around the H-bond axis. The fast (about 1 ps) relaxation, relatively temperature independent, is associated with rototranslational coupling motion of the water molecule in a disordered cage. The origin of the fast rotational relaxation comes from "interstitial molecules" coupling with the center water molecule.