The short-time dynamics of molecular liquids. Instantaneous-normal-mode theory

Büchner, M; Ladanyi, Branka M.; Stratt, Richard M.

doi:10.1063/1.463370

Cited by 242 publications

(92 citation statements)

References 41 publications

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“…Previous studies have indicated that taking into account only stable (real frequency) INMs does not lead to appreciable errors in the very short time dynamics of the fluid considered, and provides a better representation of the exact fluid dynamics than an all-mode treatment at longer times. [36][37][38] Here we follow the same practice and represent the water dynamics using only the subset of stable INMs. In the discussion below we refer to these modes as 'phonons', keeping in mind the limited applicability of this term in the present context.…”

Section: Transmission Through Water Described By Instantaneous Normalmentioning

confidence: 99%

“…Instead we use previous work that indicate that on timescales shorter than ~100fs the nuclear dynamics of different solvents, including water, can be reliably described in the instantaneous normal mode picture. [35][36][37][38] In this approach each static liquid configuration sampled from the equilibrium ensemble is used as a reference for expanding the nuclear potential up to second order in the deviations of the nuclear coordinates from this reference configuration. Diagonalizing the Hessian matrix associated with the second order derivatives of the nuclear potential about this reference point then yields a set of 'instantaneous normal modes' (INMs) whose dynamics describes the short time evolution of the liquid about that configuration.…”

Section: Transmission Through Water Described By Instantaneous Normalmentioning

confidence: 99%

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Inelastic effects in electron tunneling through water layers

Galperin

Nitzan

2001

The Journal of Chemical Physics

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Calculations of tunneling matrix elements associated with electron transfer through molecular environments are usually done for given frozen nuclear configurations; the underlying assumption being that nuclear motions are slow relative to the timescale of a tunneling event. This paper examines this issue for the case of electron tunneling through water. The motivation for this study is a recent calculation [Peskin et al, J. Chem. Phys. 111, 7558 (1999)] that indicates that electron tunneling through water may be enhanced by tunneling resonances in the range of ~1eV below the vacuum barrier, and finds that the lifetimes of such resonances are in the 10fs range, same order as OH stretch periods. Our calculation is based on the absorbing-boundary-conditions-Green's-function (ABCGF) method and proceeds in two steps. First we consider the effect of a single symmetric OHstretch mode on electron tunneling in an otherwise frozen water environment and establish that the inelastic tunneling probability is small enough to justify an approach based on perturbation theory limited to single phonon transitions. Next we note that on the short timescale of a tunneling event, even under resonance conditions, water nuclear dynamics may be represented in the instantaneous normal modes picture. We generalize the ABCGF method to take into account low order inelastic scattering from a continuum of such harmonic normal modes. We find that near resonance the total inelastic transmission probability is of the same order as the elastic one, and may lead to an additional ~20-40% enhancement of the overall transmission in the range of up to 1eV below the vacuum barrier. The absolute energy exchange is small, of the order of 1% of the incident electron energy. Surprisingly, we find that the main contribution to the inelastic transmission is associated with energy transfer into the rotational-librational range of the water instantaneous normal mode spectrum.

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Section: Transmission Through Water Described By Instantaneous Normalmentioning

confidence: 99%

Section: Transmission Through Water Described By Instantaneous Normalmentioning

confidence: 99%

Inelastic effects in electron tunneling through water layers

Galperin

Nitzan

2001

The Journal of Chemical Physics

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“…To conclude, we briefly compare our description with the Instantaneous Normal Mode (INM) analysis of supercooled liquids [21]. A fruitful idea in the INM context is to relate diffusion to the average fraction of negative eigenvalues of the potential energy Hessian [22]: instantaneous unstable modes are interpreted as the contribution of regions with negative curvature sampled by the system in crossing potential energy barriers.…”

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confidence: 99%

Fragile vs . strong liquids: A saddles-ruled scenario

Cavagna¹

2001

Europhys. Lett.

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PACS. 61.20.Gy -Theory and models of liquid structure. PACS. 64.70.Pf -Glass transitions.Abstract. -In the context of the energy landscape description of supercooled liquids, we propose an explanation for the different behaviour of fragile and strong liquids. Above Goldstein's temperature Tx, diffusion is interpreted as a motion in the phase space among saddles of the potential energy. Two mechanisms of diffusion then arise: mechanism A takes place when the system overcomes potential energy barriers along stable uphill directions, while mechanism B consists in finding unstable downhill directions out of a saddle. Depending on the mutual efficiency of A and B, the usual classification of liquids in fragile and strong is recovered. Moreover, this scenario naturally predicts the possibility of a fragile-to-strong crossover when lowering the temperature.After the seminal paper by Goldstein in 1969 [1], it has become customary to think of the dynamical evolution of supercooled liquids in terms of motion of the state point of the system upon its rugged potential energy surface [2]. More precisely, at low temperatures, but above the glass transition, the diffusion of a system at equilibrium can be interpreted as the result of two different processes: the thermal relaxation into basins defined by the many minima of the potential energy (intra-basin relaxation) and the hopping from basin to basin by crossing potential energy barriers (inter-basin relaxation) [1].Crucial condition for this description to be correct is that the two relaxation times τ intra and τ inter are well separated, that is τ inter ≫ τ intra : if these time scales are of the same order, it is not sensible to discriminate between the thermalization inside a minimum and the hopping among different minima. Indeed, the very requirement of the separation between these two time scales led Goldstein in [1] to define and estimate a crossover temperature T x , above which this hopping-relaxation description is no longer valid, since τ intra and τ inter are not well separated. Below T x , on the other hand, crossing of potential energy barriers by thermal activation becomes the primary mechanism of diffusion. The Goldstein temperature T x is in general higher than the glass transition temperature T g : in the interval T g < T < T x a supercooled liquid is still in equilibrium on experimental times and can be described in terms of relaxation into basins and thermally activated hopping among them.Some authors [3,4] have subsequently interpreted T x as the temperature below which the ideal Mode Coupling Theory (MCT) [5] breaks down. This is because MCT is considered c EDP Sciences

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“…Moreover the INM approach has been exploited to describe the dynamical features of molecular liquids (e.g. diatomic Lennard-Jones [6], CS 2 [7]) including hydrogen bonded systems like water [8,3]. On the other hand recent molecular dynamics (MD) simulations of HF have revealed peculiar dynamical features, e.g.…”

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confidence: 99%

“…As a final remark we wish to point out that the clear separation between translational and rotational components can be considered as a print of hydrogen bonded systems. In fact it is present in water and HF but not in a linear Lennard-Jones diatomic which has been investigated in [6]. To analyse in more detail the structure of the INM we need to determine which molecules participate in each given mode.…”

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confidence: 99%

Instantaneous Normal Mode Analysis of Liquid HF

Garberoglio

Vallauri

2000

Phys. Rev. Lett.

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We present an Instantaneous Normal Modes analysis of liquid HF aimed to clarify the origin of peculiar dynamical properties which are supposed to stem from the arrangement of molecules in linear hydrogen-bonded network. The present study shows that this approach is a unique tool for the understanding of the spectral features revealed in the analysis of both single molecule and collective quantities. For the system under investigation we demonstrate the relevance of hydrogen-bonding "stretching" and fast librational motion in the interpretation of these features.PACS numbers: 62.60.+v, 63.20.Pw In recent years the analysis of Instantaneous Normal Modes (INM) of normal and supercooled liquids has given a sound improvement to the understanding of the microscopic processes underlying the dynamical properties of these systems. Applications of the method are to be found in the calculation of macroscopic quantities through the knowledge of the density of states (e.g. the diffusion coefficient [1]) and in the interpretation of atomic motion through the inspection of the eigenvectors [2][3][4]. This last method appears to be a unique tool for the interpretation of dynamical features (single molecule or collective) of a disordered system in terms of correlated motions of its constituents. In particular it is interesting to explore how the presence of locally ordered units is reflected in the time behaviour of, for example, the velocity autocorrelation function (VACF). Attempts in this direction have to be found in the analyses of: i) the correlation function of the projection of the centre of mass (CoM) velocity of water molecules along the directions of the normal coordinates of a cluster of three molecules [5]; ii) the projection of INM eigenvectors onto the totally symmetric displacement coordinates of ZnCl 2− 4 tetrahedral units [2]. Moreover the INM approach has been exploited to describe the dynamical features of molecular liquids (e.g. diatomic Lennard-Jones [6], CS 2 [7]) including hydrogen bonded systems like water [8,3]. On the other hand recent molecular dynamics (MD) simulations of HF have revealed peculiar dynamical features, e.g. a peak at ≃ 50 ps −1 in the spectra of both collective and single molecule correlation functions [9,10]. In the collective longitudinal and transverse current spectra this mode appears to have an optical-like character, since its frequency is found to be independent of the wavevector. For the interpretation of the CoM VACF spectrum this peak has been related to the relative motion of two nearest neighbour molecules [10]. Since nearest neighbours are also hydrogen bonded it is tempting to assign this dynamical feature to hydrogen bonding "stretching". As a matter of fact HF molecules have been demonstrated to form irregular zig-zag chains of different size, being this peculiar clustering favoured by the geometry of the molecule and the strong electrostatic interaction [11]. A dynamical characterisation of these ordered units has not yet been given, so that any assignment of the sp...

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The short-time dynamics of molecular liquids. Instantaneous-normal-mode theory

Cited by 242 publications

References 41 publications

Inelastic effects in electron tunneling through water layers

Inelastic effects in electron tunneling through water layers

Fragile vs . strong liquids: A saddles-ruled scenario

Instantaneous Normal Mode Analysis of Liquid HF

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