Translational Density of States Derived from Short-Time Relative Motions

2000

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Hydrogen bond and residence dynamics of ion-water and water-water pairs in supercritical aqueous ionic solutions: Dependence on ion size and densityThe 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.

mentioning

confidence: 63%

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

2000

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“…The specific correlation function B(r 0 ;t) we use 15 is the projection of R i j (t) over u(0), the unit vector of the direction parallel ͑longitudinal dynamics͒ or perpendicular ͑transverse dynam-ics͒ to R i j (0), i.e.,…”

Section: Theoretical Recalls and MD Simulationsmentioning

confidence: 99%

Negative contributions in the velocity correlation function of supercooled liquid water

2004

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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).

“…1,2 The method was used to derive a meaningful description of the translational motions in liquid supercooled water (T ϭ243 K) and is here applied to liquid water at the melting and boiling points ͑273 and 373 K͒. Many studies have been devoted to the microscopic dynamics of water and to the relaxation mechanisms that bring a liquid towards a glass behavior as the temperature decreases and the temperature of the glass transition is reached.…”

Section: Introductionmentioning

confidence: 99%

Temperature evolution of the translational density of states of liquid water

2001

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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.