Molecular dynamics simulation of liquid water along the coexistence curve: Hydrogen bonds and vibrational spectra

Abstract

Structure and dynamic properties of liquid water at temperatures between 298 and 523 K and densities between 0.75 and 1.20 g/cm3 have been investigated by molecular dynamics simulation. A flexible simple point charge potential has been asssumed for interactions. The hydrogen bonding structure in the different simulated states as well as the influence of the hydrogen bonds on the dynamic properties (self-diffusion coefficients, vibrational spectra) is discussed. Special attention is paid to the intermolecular vibrational spectrum (10–400 cm−1). It has been corroborated that the band around 200 cm−1 can be attributed to intermolecular O–O stretching vibrations of pairs of H-bonded bounded molecules. On the contrary, molecular dynamics results indicate that the band close to 50 cm−1 is independent of the existence of hydrogen bonds but depends on the density and temperature of the system. It is suggested that it is simply associated with vibrations of molecules in the cage formed by their neighbors. Shifts of librational and stretching bands as a function of the thermodynamic state are highly correlated with changes in the percentage of hydrogen bonded molecules.