Alessandro Ercoli

Comment on “An interpretation of the low-frequency spectrum of liquid water” [J. Chem. Phys. 118, 452 (2003)]

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.

ON THE SHORT TIME MOTION OF HYDROGEN-BONDED MOLECULES IN SUPERCOOLED WATER

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 veloc...

Temperature evolution of the translational density of states of liquid water

The molecular dynamics technique is used to study the relative dynamics of tagged pairs of molecules and to derive the related translational density of states (DOS) 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 d...

Excess of low-frequency modes in Lennard-Jones systems

Argon dynamics is simulated in the glassy state at the temperature of 20 K. A fast quenching is used to study the slow time evolution from the glass to the crystalline state. Observation of the crystallization process reveals the growth of temperature, a rapid change in the mean square displacement, and strong variations in the relative pair dynamics. Our main results are: (a) detecting the presence of a low-frequency mode in the pair dynamics of the glass; (b) showing that this mode is responsible for the intensity excess at low frequency in the power spectrum (compared to that of the crystal); and (c) showing that it decreases more and more as the crystallization proceeds.

Negative contributions in the velocity correlation function of supercooled liquid water

The translational dynamics of supercooled and normal liquid water is investigated via a specific correlation function DeltaB with the aim of explaining the behavior of the centers of mass velocity correlation function (VCF). DeltaB is divided into diffusive and nondiffusive parts that yield separated contributions to the VCF, namely an Enskog-type diffusive one, modeled by an exponential function, and a nondiffusive one, made up by damped oscillations of a vanishing time integral. In the translational density of states (DOS), the oscillations yield the bands at omega(1) congruent with 50 cm(-1), omega(3) congruent with 240 cm(-1) (the two well-known experimental bands of the Raman spectra) and omega(2) congruent with 160 cm(-1) (the Einstein frequency of the liquid). It is shown that the chief negative lobe of the VCF is mainly due to the DOS component at the lowest frequency omega(1). The study of the relative pair dynamics shows that this lobe is due to the transverse dynamics, while the longitudinal one determines the fast DOS component at omega(3). The presence of a negative tail is highlighted. Its contribution extends beyond the region of the fast dynamics (t<0.7 ps) up to about 1.5 ps and is due to a low-frequency oscillating mode that produces a low-frequency DOS band centered at about omega(0)=20 cm(-1).

Vibrational motions of hydrogen bonds from the short time relative pair dynamics

The short time dynamics of pairs of water molecules, initially lying in the first coordination shell, is investigated via molecular dynamics simulation. The introduction of the generalized time-dependent pair distribution function allows to obtain a relationship between the dynamics and the local structure. The relationship explains the different short time behaviors between the hydrogen-bonded molecules and the structural defects, and the lack of the free flight time dependence of the mean-square distance. The centers of mass vibrational motion of hydrogen-bonded molecules influences the relative pair dynamics beyond the short time expansion. An approach, based on the instantaneous normal modes theory, is proposed to derive the vibrational motion of the hydrogen bonds. Its general applicability is stressed and the particular relevance for studying systems whose dynamics is determined by strong oriented interactions is suggested.

Short time expansion of the relative motion of molecular pairs. Application to water

A detailed analysis of the short time expansion of the relative mean square displacement and mean square distance of pairs is performed for molecular fluids. A theoretical expression which relates the t2 coefficient of the expansion to the local structure of the fluid is obtained. Good agreement between theoretical predictions and simulated data is found in the case of supercooled water.