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Vibrational Features of Water at the Low-Density/High-Density Liquid Structural Transformations

A structural transformation in water upon compression was recently observed at the temperature $T=277$~K in the vicinity of the pressure $p \approx 2\;000$~Atm [R.M. Khusnutdinoff, A.V. Mokshin, J. Non-Cryst. Solids \textbf{357}, 1677 (2011)]. It was found that the transformations are related with the principal structural changes within the first two coordination shells as well as the deformation of the hydrogen-bond network. In this work we study in details the influence of these structural transformations on the vibrational molecular dynamics of water by means of molecular dynamics simulations on the basis of the model Amoeba potential ($T=290$~K, $p=1.0 ÷10\;000$~Atm). The equation of state and the isothermal compressibility are found for the considered ($p$,$T$)-range. The vibrational density of states extracted for $THz$-frequency range manifests the two distinct modes, where the high-frequency mode is independent on pressure whereas the low-frequency one has the strong, non-monotonic pressure-dependence and exhibits a step-like behavior at the pressure $p \approx 2000$~Atm. The extended analysis of the local structural and vibrational properties discovers that there is a strong correlation between the primary structural and vibrational aspects of the liquid-liquid structural transformation related with the molecular rearrangement within the range of the second coordination shell.