Collective dynamical properties of molten salts: molecular dynamics calculations on sodium chloride

Abstract

We present the results of molecular dynamics calculations on a system simulating molten NaCl at a temperature of 1090 K. Attention has been focused on the study of the collective modes, as described by the auto-correlation functions of the longitudinal and transverse components of the currents of mass and charge. Explicit expressions are given for the coefficients in the small-time Taylor development of the autocorrelation functions and these, together with data on the static structure factors, are used in analysing the current fluctuations and their spectra in terms of memory functions. The memory function has the same basic structure in all cases, consisting of a short-lived initial decay and a long-lived quasi-exponential tail. Inclusion of the tail is essential in order to achieve quantitative agreement with the measured spectra, particularly at small wavenumbers. To that extent our results are consistent with calculations on simpler systems, suggesting that the dynamical events which contribute to the tail in the memory functions are a feature characteristic of liquids in general. The relation to neutron-scattering experiments is also discussed. It is shown, in particular, that a propagating charge density fluctuation of the type seen in the molecular dynamics results is likely to be undetectable in a neutron experiment, except in particularly favourable circumstances.