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The temperature dependence of the diffusion coefficient of liquid alkali metals is studied along a constant volume line and the liquid-gas coexistence curve as well. Molecular-dynamics simulations are used to compute the velocity autocorrelation functions from which the self-diffusion coefficients are deduced. Along both lines of transformation, the Arrhenius law appears to be unable to fit the temperature dependence whatever be the alkali element. The numerical results obtained for different values of density and temperature are in good agreement with experiments near the melting point and can be interpreted in terms of a power law at higher temperatures.

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Thermodynamic properties of the Lennard-Jones (LJ) fluid are investigated by studying a system of particles interacting with a potential of hard-core plus attractive Yukawa tail (HCY). Due to the similarity of the LJ and the HCY potentials in their overall form, it is worthwhile seeking to approximate the LJ potential in much the same way that the hard-sphere reference potential has been used. The study consists in describing the thermodynamics of the LJ fluid in terms of the equivalent HCY system, whose properties are known accurately, by means of mapping the thermodynamic quantities for the HCY potential parameters. The method is feasible owing to a convenient analytical expression for the Helmholtz free energy in the mean-spherical approximation expanded in powers of the inverse temperature. Two different procedures are used to determine the parameters of the HCY potential as a function of the thermodynamic states: one is based on the simultaneous fits of pressure and internal energy of the LJ system, and the other uses the concept of collision frequency. The reasonable homogeneity of the results in both procedures of mapping makes the HCY potential a very good reference system whose theoretical expressions can be used confidently to predict the thermodynamic properties of systems with more realistic potentials.PACS numbers: 61.20. Gy, 65.90.+i, UDC 536.632, 538.953

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