The comparison between the translational densities of states of water and argon suggests that the water bands at about 60 and 240 cm−1 reflect the transverse and longitudinal dynamics, respectively. The water–argon similarity and the role of the hydrogen bonds in producing more intense and sharp bands are highlighted. Our interpretation partially contradicts that of the authors of the title article.

The short time dynamics of tagged pairs of molecules that, at the initial time, are in the first coordination shell, is investigated in supercooled liquid water at 245 K by using the molecular dynamics technique with the four-points transferable intermolecular potential of Jorgensen et al. [J. Chem. Phys. 79, 926 (1983)]. The instantaneous normal mode approach and the results of the local structure investigations are exploited to build up a correlation function of the relative displacements that represents the projection of normal modes along the initial center of mass separation vector. By imposing simple constraints to the initial dynamical conditions, localized damped oscillations of the centers of mass are detected along the hydrogen bond directions. The corresponding density of states shows a maximum around the frequency of 230 cm−1 and its shape agrees with the frequency contributions expected from the translational phonon branches of ice. Total and radial correlation functions of the relative velocity are also computed. The radial component is dominated by oscillations at the frequency of 230 cm−1; it compares fairly well with that derived from the translational density of states, thus supporting the reliability of the employed method. A slower component of motion, absent along the hydrogen bond direction, is derived; in the density of states, it produces a contribution peaked around 50 cm−1.

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The influence of temperature and density functional models in ab initio molecular dynamics simulation of liquid waterThe molecular dynamics technique is used to study the relative dynamics of tagged pairs of molecules and to derive the related translational density of states ͑DOS͒ of liquid water at 243, 273, and 373 K. The modes that compose the short-time dynamics of centers of mass are obtained. The dynamical quantities studied are characterized by a fast-time decay followed by a plateau whose height increases with the temperature and with the initial pair separation. The plateau is attributed to the nonharmonic motions and its height is related to the pair relative diffusion coefficient. An exponential relaxation is used to represent the way the system follows to reach the diffusive behavior; the derived relaxation times agree with those reported in the literature describing the fast translational dynamics. The frequencies of the other short-time modes are related to the main frequencies of the solid, while the mode damping is analyzed in terms of the damped harmonic oscillator model; it is found that the Gaussian damping gives a better fit to the DOS than the exponential one. The temperature evolution of the various modes is investigated and related to the microscopic pair dynamics. In particular, the modes at low frequencies ͑Х50 cm Ϫ1 ͒ are absent in the vibrations along the hydrogen bond ͑longitudinal modes͒; they are present in the transverselike dynamics of other pair states. The temperature increase produces the intensity decrease of the 50-cm Ϫ1 band and the pile-up of the DOS intensity towards zero frequency. The decay constants of these two effects have a different temperature dependence, which confirms the oxygen bending nature of the 50-cm Ϫ1 band and its independence on the relaxational-like dynamics.