  It was suggested in the literature that the self-diffusion coefficient of
simple fluids can be approximated as a ratio of the squared thermal velocity of
the atoms to the "fluid Einstein frequency," which can thus serve as a rough
estimate of the friction (momentum transfer) rate in the dense fluid phase. In
this article we test this suggestion using a single-component Yukawa fluid as a
reference system. The available simulation data on self-diffusion in Yukawa
fluids, complemented with new data for Yukawa melts (Yukawa fluids near the
freezing phase transition), are carefully analyzed. It is shown that although
not exact, this earlier suggestion nevertheless provides a very sensible way of
normalization of the self-diffusion constant. Additionally, we demonstrate that
certain quantitative properties of self-diffusion in Yukawa melts are also
shared by systems like one-component plasma and liquid metals at freezing,
providing support to an emerging dynamical freezing indicator for simple soft
matter systems. The obtained results are also briefly discussed in the context
of the theory of momentum transfer in complex (dusty) plasmas.
