Collective oscillations in thermal equilibrium were shown to represent only a small fraction of the degrees of freedom of a liquid metal, the important fraction being associated with uncorrelated motions of the atoms. The fundamental importance of these oscillations to the theory of diffusion was justified here. They could be explained within the framework of a collective coordinate formulation, where the relevant Fourier components of the pair-potential were only those corresponding to the wave-vectors of the collective modes. Since the matrix elements for atomic scattering processes were proportional to the Fourier coefficients, it followed that, in this formulation, the only important processes were collisions; where the wave-vector change of an atom was a wave-vector of the collective modes. A reduction in the number of these modes, as produced by a temperature rise, implied a reduction in the scattering processes and then of the resistance presented by the liquid to the motion of a diffusing atom. This explained the strong increase, with temperature, of the self-diffusion coefficient; as observed in liquid Li. Perturbation theory could be used to account for those Fourier components, which were neglected in the above formulation. This made the results fully consistent with the present theory of diffusion.

Diffusion and Collective Motions in Liquid Metals. M.Omini: Philosophical Magazine, 2007, 87[33], 5249-78