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The diffusion coefficients of neutron rich nuclei in crystallizing whitedwarf (WD) stars are essential microphysics input for modeling the evolution ofthe composition profile. Recently, molecular dynamics simulations have beenused to compute diffusion coefficients for realistic mixtures of C-O and O-NeWDs with many trace nuclides that could be important sedimentary heat sourcessuch as $^{22}$Ne, $^{23}$Na, $^{25}$Mg, and $^{27}$Mg. In this brief note, Irepeat these simulations but now include $^{56}$Fe. I find that for the largecharge ratios involved in these mixtures the empirical law developed in ourearlier work tends to under-predict diffusion coefficients in the moderatelycoupled regime by 30 to 40 percent. As this formalism is presently implementedin the stellar evolution code MESA, it is important for authors studyingmixtures containing heavy nuclides like $^{56}$Fe to be aware of thesesystematics. However, the impact on astrophysics is expected to be small.

Diffusion Coefficients of $^{56}$Fe in C-O and O-Ne White Dwarfs