Spectral signatures of the diffusional anomaly in water

Power spectra for various tagged particle quantities in bulk extended simple point charge model water [H. J. C. Berendsen, J. R. Grigera, and T. P. Straatsma, J. Phys. Chem. 91, 6269 (1987)] are shown to have a regime with 1f(alpha) dependence on frequency f with alpha lying between 1 and 1.5 if the dynamical changes in the particular observable are sensitive to the multiple time-scale behavior of the hydrogen-bond network. The variations in mobility associated with the diffusional anomaly are mirrored in the scaling exponent alpha associated with this multiple time-scale behavior, suggesting that monitoring of 1f(alpha) behavior is a simple and direct method for linking phenomena on three distinctive length and time scales: the local molecular environment, hydrogen-bond network reorganizations, and the diffusivity. Our results indicate that experimental studies of supercooled water to probe the density dependence of 1f(alpha) spectral features, or equivalent stretched exponential behavior in time-correlation functions, will be of interest.