  Diffusion coefficients are essential microphysics input for modeling white
dwarf evolution, as they impact phase separation at crystallization and
sedimentary heat sources. Present schemes for computing diffusion coefficients
are accurate at weak coupling ($\Gamma \ll 1$), but they have errors as large
as a factor of two in the strongly coupled liquid regime ($1 \lesssim \Gamma
\lesssim 200$). With modern molecular dynamics codes it is possible to
accurately determine diffusion coefficients in select systems with
percent-level precision. In this work, we develop a theoretically motivated law
for diffusion coefficients which works across the wide range of parameters
typical for white dwarf interiors. We perform molecular dynamics simulations of
pure systems and two mixtures that respectively model a typical-mass C/O white
dwarf and a higher-mass O/Ne white dwarf, and resolve diffusion coefficients
for several trace neutron-rich nuclides. We fit the model to the pure systems
and propose a physically motivated generalization for mixtures. We show that
this model is accurate to roughly 15% when compared to molecular dynamics for
many individual elements under conditions typical of white dwarfs, and is
straightforward to implement in stellar evolution codes.
