  The diffusion coefficients of neutron rich nuclei in crystallizing white
dwarf (WD) stars are essential microphysics input for modeling the evolution of
the composition profile. Recently, molecular dynamics simulations have been
used to compute diffusion coefficients for realistic mixtures of C-O and O-Ne
WDs with many trace nuclides that could be important sedimentary heat sources
such as $^{22}$Ne, $^{23}$Na, $^{25}$Mg, and $^{27}$Mg. In this brief note, I
repeat these simulations but now include $^{56}$Fe. I find that for the large
charge ratios involved in these mixtures the empirical law developed in our
earlier work tends to under-predict diffusion coefficients in the moderately
coupled regime by 30 to 40 percent. As this formalism is presently implemented
in the stellar evolution code MESA, it is important for authors studying
mixtures containing heavy nuclides like $^{56}$Fe to be aware of these
systematics. However, the impact on astrophysics is expected to be small.
