  A variation principle for mass transport in solids is derived that recasts
transport coefficients as minima of local thermodynamic average quantities. The
result is independent of diffusion mechanism, and applies to amorphous and
crystalline systems. This unifies different computational approaches for
diffusion, and provides a framework for the creation of new approximation
methods with error estimation. It gives a different physical interpretation of
the Green function. Finally, the variational principle quantifies the accuracy
of competing approaches for a nontrivial diffusion problem.

Statistical Mechanics;Materials Science