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We use the saddle-approach, recently introduced in the numerical investigation of simple model liquids, in the analysis of a mean-field solvable system. The investigated system is the k-trigonometric model, a k-body interaction mean field system, that generalizes the trigonometric model introduced by Madan and Keyes [J. Chem. Phys. 98, 3342 (1993)] and that has been recently introduced to investigate the relationship between thermodynamics and topology of the configuration space. We find a close relationship between the properties of saddles (stationary points of the potential energy surface) visited by the system and the dynamics. In particular the temperature dependence of saddle order follows that of the diffusivity, both having an Arrhenius behavior at low temperature and a similar shape in the whole temperature range. Our results confirm the general usefulness of the saddle-approach in the interpretation of dynamical processes taking place in interacting systems. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using a realistic flexible molecule model of the fragile glass former orthoterphenyl, we calculate via molecular dynamics simulation the collective dynamic structure factor S(Q,ω), recently measured in this system by inelastic x-ray scattering. The comparison of the simulated and measured dynamic structure factor, and the study of the S(Q, ω) in an extended momentum (Q), frequency (ω), and temperature (T) range allows us (i) to conclude that the utilized molecular model gives rise to S(Q, ω) in agreement with the experimental data, for those thermodynamic states and Q values where the latter are available; (ii) to confirm the existence of a slope discontinuity on the T dependence of the sound velocity that, at finite Q's, takes place at a temperature Tx higher than the calorimetric glass transition temperature T[sub g]; (iii) to find that the value of T[sub x] is Q-dependent and that its Q→0 limit is consistent with T[sub g]. The latter finding is interpreted within the framework of the current description of the dynamics of supercooled liquids in terms of exploration of the potential energy landscape. [ABSTRACT FROM AUTHOR]

Detailed computer simulation results of several static and dynamical properties of water, obtained by using a realistic potential model proposed by Jorgensen et al., in the supercooled region, at densities well below the coexistence curve, are reported. We have analyzed the structural properties by evaluating the volume distributions of Voronoi polyhedra as well as angular and radial distributions of molecular clusters. In particular, the homogeneity of the system has carefully been checked. The investigated dynamical properties mainly concern the density and temperature dependence of the diffusion coefficient. The results are compared both with previous simulations, performed with different models, and with experimental findings. Some differences stress the fact that the conclusions drawn on the physical process underlying density and temperature behavior, can strongly be influenced by the use of different potential models. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations of liquid water at various temperatures is reported. The autocorrelation functions of linear and angular velocities and of the first and second rank rotation matrices are studied and compared with previous data obtained using different potential models. Comparisons with the results of molecular dynamics simulations of heavy water and hydrogen sulfide are also shown. [ABSTRACT FROM AUTHOR]

The dynamic structure factor S(Q, ω) of liquid hydrogen fluoride is investigated by inelastic X-ray scattering at T=239 K in a momentum transfer region between 2 and 31 nm-1. The extracted apparent sound velocity c(Q) shows a transition due to the structural or α relaxation with a relaxation time τα(Q) on the subpicosecond (less than 10-12 s) time scale. Nevertheless, from a comparison with existing molecular dynamics simulations, it arises that a second faster relaxation is present. This allows us to discuss our experimental results in terms of a two-relaxation-process scenario. [ABSTRACT FROM AUTHOR]

The collective dynamics of a simple metal (lithium) have been numerically investigated by molecular dynamics and normal mode analysis. A generalized hydrodynamic framework allowed us to describe the system in different thermodynamic phases by the same model based on two relaxation processes. At the melting point we find good agreement with recent inelastic X-ray scattering results, while below the melting point, and ultimately in the glassy state, we find evidences of a 'harmonic' relaxation as recently found in Lennard-Jones systems. [ABSTRACT FROM AUTHOR]

We present a computer simulation study of a monatomic glass model. The investigated system consists of up to 32 000 'argon' atoms, interacting via a Lennard-Jones potential. Its dynamical properties have been investigated both by the time evolution of the atomic trajectories during molecular dynamics runs at low temperature and by the normal mode analysis (NMA) in the inherent configuration. In the NMA, we used both the direct diagonalization of the dynamical matrix and the spectral moment method. The vibrational density of states (DOS), the dynamic structure factors, the Raman spectrum and the spatial patterns of the eigenmodes have been calculated. The Raman spectrum shows the characteristic bump, that is the so-called 'boson peak', at frequencies where the DOS presents an excess of states with respect to the Debye behaviour. By the analysis of the previous dynamical quantities and from the direct inspection of the pattern of the eigenvectors, we derive a consistent picture of the atomic dynamics in the boson peak energy region. [ABSTRACT FROM AUTHOR]

We present an experimental setup to study terahertz dynamics in fluids under high pressure, employing inelastic x-ray scattering and diamond anvil cell techniques. The use of a carefully designed vacuum chamber and the minimization and control of sources of parasitic scattering allowed circumventing previous limitations due to important empty cell contributions to the scattering signal. The successful implementation of our setup is demonstrated in the case of supercritical fluid argon, for which a full viscoelastic analysis yields the dispersion relation of sound waves, the generalized heat capacity ratio, and longitudinal viscosity. Our results are in excellent agreement with available experimental observables and molecular dynamics simulations. [ABSTRACT FROM AUTHOR]