Solid diffusion model for solid state solution implementation error

[lightningclaw001]

Eq 57 in the MPET paper is implemented incorrectly in the software, should either be implemented as
F = - D* Dfunc/T * (dmu/dc) \nabla c if assuming not dilute solution model (and dc/dmu \approx Dfunc/T)
or
F = - D * \nabla c. (I am leaning towards implementing this one since this is more physically accurate for comparisons with other software and it is the actual definition of the flux)
@d-cogswell what do you think?

[d-cogswell]

Ooh, good catch. The last part of Eq 57 is wrong in the paper, the 1/RT gets canceled by the units of \mu when non-dimensionalized. Go with F = - D * \nabla c, it's faster to evaluate and we don't know what mu is for many models.

[d-cogswell]

Keep the Dfunc(c), which shows up in some models.

[lightningclaw001]

Yeah, I will keep the Dfunc since it is good for CHR. I think technically equation 57 is right and they just eat up the dmu/dc term in the Dchem though, but the implementation in MPET seems wrong