Hi!
Well, again that's something that could do with a little bit more documentation...
Actually, for backward compatibility reasons, by default MICRESS only uses the diagonal terms of the diffusion matrix (looking back on it, it might not be the most obvious choice, but there you are...), so "database_*" only causes the diagonal term to be calculated from a database by TQ. To get the complete Fick-Onsager's equation you need to specify "multi".
This part is a bit convoluted, so it's perhaps better if I go over the whole thing:
- "diff" -> only diagonal term, user defined.
- "database_global" -> only one "global" diagonal term, from the database.
- "database_local" -> only diagonal term, from the database, with linear dependency on the local composition.
- "multi" -> full Fick-Onsager's equation with "global" coefficitents from the database.
- it is also possible to be more specific, and add (after "multi") a string made of "n", "d", "g" and "l" for each component. "n" meaning "no diffusion" (i.e.: do not take into account the influence of the gradient of this component on the flux), "d" for "user defined" diffusion coefficient (which should then be input), "g": "global" (i.e.: one only per phase and pair of components) diffusion coefficient from the database, and "l": diffusion coefficients taking (linearly) into account the influence of local composition.
The influence of local composition on the diffusion coefficients within one phase is taken into account via linear interpolation. This approach was suggested by John Agren and Lars Hoglund who considered it an appropriate estimate "in most cases".
Well... I think it sums it up, if I missed something or went too fast over it, please let me know.
Philippe
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original message from Philippe