[Ifeffit] Using more than one E0

[Scott Calvin]

Matt said:
>
>Hmm, really??? I'm not sure of that.  Even disregarding the
>different k-dependences of E0 and C3, I'm not sure how using
>different E0's for different paths (especially with the notion that
>they are to be applied to different coordination species) could mask
>a third cumulant for a particular path. Do you have an example?

I never followed up in a case where I've seen it (except maybe to ask 
a question of the speaker), but I'm sure I've seen talks where 
someone revealed they routinely use a different E0, not for different 
species, but for the first coordination shell. That's the case that 
sets off my alarm bells. If someone is using different E0's to 
different atom types, as Shelly suggests, then I am much less likely 
to suspect they are really trying to mask a C3 effect.

>As discussed earlier, using different E0's for different shells does
>have some physical interpretation: that the single, flat energy
>origin from the muffin tin approximation is incomplete.  That's
>about as physical as the first E0 and S02.  My guess is that a
>second E0 is about as likely to be needed as non-zero C3 unless you
>have purposely disordered (that is, hot) samples.

I've successfully modeled thermal expansion in fcc metals below room 
temperature by using a C3, and it requires nearest-neighbor C3's that 
are significantly nonzero. So although I won't dispute that there may 
be common materials where more than one E0 is needed for high 
accuracy (oxides?), it also appears to be true that everyone's 
favorite practice problem (good old copper), requires a non-zero 
nearest-neighbor C3 for high accuracy fits.

>It's certainly easy enough to add fudge factors to a fit. The hope
>is that the model has some physical meaning or that a reviewer would
>catch abuses!

Amen! :)

--Scott Calvin
Sarah Lawrence College