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A relaxation process, with the associated phenomenology of sound attenuation and sound velocity dispersion, is found in a simulated harmonic LennardJones glass. We propose to identify this process with the so called microscopic (or instantaneous) relaxation process observed in real glasses and supercooled liquids. A model based on the memory function approach accounts for the observation, and allows to relate to each others: 1) the characteristic time and strength of this process, 2) the low frequency limit of the dynamic structure factor of the glass, and 3) the high frequency sound attenuation coefficient, with its observed quadratic dependence on the momentum transfer.

A molecular-dynamics simulation is presented which explores the microscopic dynamics of a monatomic model glass. The investigated systems consist of up to 32000 atoms interacting via a Lennard-Jones potential. The normal-modes analysis has been used to determine the pattern of atomic displacements. Except for the highest frequencies, all the vibrational modes are found to be delocalized. In the lowest-frequency region the pattern of atomic displacements associated with a given eigenmode is composed by an uncorrelated random component plus well-defined sinusoidal-like waves. The two components are of comparable amplitudes.

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