[Ifeffit] a question about the white line

Matt Newville newville at cars.uchicago.edu
Sun Dec 12 23:03:05 CST 2004


Hi John, Josh,

>   Certainly if a spline can do a good job of fitting the mu_0, then the current
> approach is more or less ok.  However in systems with strong white lines
> mu_0 can be strongly peaked and hence very difficult to fit with a few
> spline points. The broadening function lamda(E) can be constrained to be small. 
> Also, for cases with large whitelines, the location of e0 is difficult to
> fit - the true e0 can lie well below the edge jump.

Oh, maybe I misunderstood. Is lambda(E) in
>     mu_0 = mu^thy_0(E,E_0,Gamma)[1 + lambda(E)]

just a broadening term or a highly adjustable spline??  I guess
the whole question is how much freedom this function has.

It definitely sounds interesting to use a theoretical mu_0,
especially for challenging white lines, but it's not clear how to
best make this accessible to users.  I'll probably have to think
about this some more, but it would be nice to know the mechanics
of how you're doing it, and how well you need to know the
structure before you start.

>    In our view, mu0 does not depend very much on structure, since it's
> mostly determined by the local embedded atom potential. For this reason
> it's calculation would also be quite fast (only phase shifts and the
> cross-section are needed) compared to a calculation of chi(k) which
> requires multiple scattering paths.

Well, usually a standard for autobk only needs a decent guess of
the first shell.  Getting a standard does require some prior
knowledge of the system and calculation time, but I don't know
that it would be a lot more knowledge and time than would be
needed for a mu^thy_0(E).  Anyway, it would still be interesting
to compare these two (autobk using calculated chi(k) v. starting
with a mu^thy_0 and massaging it to match data).

On the other hand, it sounds as if the mu^thy_0(E) for a few dozen
(or few hundreds) challenging systems could be tabulated.  That 
would make it very fast and very simple to use!

--Matt




More information about the Ifeffit mailing list