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

Santis, A. De; Ercoli, Alessandro; Rocca, Dario

doi:10.1063/1.481410

Cited by 3 publications

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References 16 publications

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“…An average non-oscillating trend results for all the pairs of the first coordination shell. A more refined explanation, based on an instantaneous normal mode approach, is reported in [11]. Here, we would stress that the function R 2 (r 0 , ; t) is very sensitive to the local structure that the molecules probe in the first time steps of motion.…”

Section: Resultsmentioning

confidence: 88%

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

Santis

Ercoli

Rocca

2000

J. Phys.: Condens. Matter

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Section: Resultsmentioning

confidence: 88%

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

Santis

Ercoli

Rocca

2000

J. Phys.: Condens. Matter

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Temperature evolution of the translational density of states of liquid water

Santis

Ercoli

Rocca

2001

The Journal of Chemical Physics

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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.

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Relative pair dynamics in simple supercooled liquids: Longitudinal contributions

2003

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The pair dynamics of simulated argon samples is investigated at the melting (85 K), supercooled (55 K), and quenched (20 K) liquid states, and in the crystal (20 K) state. Tagged pairs, initially lying in a given shell, were divided into incoming and outgoing groups and followed along simulated trajectories. Over them, specific correlation functions deltaB(r(0);t), involving the pair separation vector projected along its initial value r(0) (longitudinal dynamics), have been evaluated. More or less pronounced oscillations are detected according to the temperature of the thermodynamic states (and, obviously, their solid or liquid nature); for each state, they depend on the initial pair distance r(0), too. The oscillations vanish after few picoseconds (fast dynamics) in the case of crystal, whereas in the supercooled liquid they decay towards a plateau, whose height increases with the temperature. It is shown that the power spectrum of deltaB(r(0);t) practically yields the same density of states (DOS) produced by the pair velocity correlation function. The deltaB(r(0);t) functions obtained from the argon crystal at 20 K produce DOS curves dominated by two main frequency contributions, at about 40 and 60 cm(-1) (Einstein and Debye frequency, respectively). Their shape is quite well reproduced by damped harmonic oscillator-like (DHO) functions vibrating at that frequencies. In liquid states, the deltaB(r(0);t) plateau, that forms after the fast DHO dynamics, accounts for the system diffusivity. The relaxation towards the plateau is modeled by an exponential function whose decay time is comparable with the average vibration period. Evidence that the liquid states conserve a certain memory of the vibrational modes of the crystal is obtained. In these states, the DHO functions at the Einstein and Debye frequencies plus an exponential function cannot reproduce the deltaB(r(0);t) shape. A pronounced shoulder, that forms around 0.5 ps, requires the contribution of a third DHO. In the DOS, it yields a band centered below 20 cm(-1) that produces low frequency DOS excess in comparison with the DOS of the crystal. This contribution is present in liquid and supercooled high temperature states and survives near the temperature of the glass transition whereas the diffusion practically vanishes.

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

Cited by 3 publications

References 16 publications

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

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

Temperature evolution of the translational density of states of liquid water

Relative pair dynamics in simple supercooled liquids: Longitudinal contributions

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