[Ifeffit] Fitting procedure

[Scott Calvin]

Hi all,

Well, I'm understanding my own views on this more clearly as the  
conversation continues, so it's been helpful to me!

There are several different issues that have floated through this  
thread, but let me focus on one aspect.

One possible fitting strategy is a shell-by-shell method, in which  
each shell is fit more or less independently. In this method, the R- 
range is chosen to try to correspond to a single shell. Because the  
first shell is often dominant in the spectrum and can "leak" quite  
significantly into other shells, it is reasonable to fit the first  
shell, and then use those parameters to constrain the leakage from the  
first shell while fitting the second shell. I've rarely used that  
strategy personally, but it seems perfectly reasonable to me, and  
there are cases where it seems like a very good way to go. There are  
no problems with free parameters with this method. For instance, if  
the first shell is fit over the R-range 1.0 to 1.5 angstroms, and the  
second from 1.5 to 2.0, each with its own parameters, then you're not  
"cheating" to use the results of the first fit to fix the parameters  
for the first shell in the second fit, as those parameters came from  
data you're not currently using. The usual idea that constraints come  
from prior knowledge is satisfied.

On the other hand, I have sometimes seen students in workshops fit the  
first shell, then extend the R-range so that it includes both shells,  
fix the first shell parameters to the results from the prior fit, and  
then do a fit to determine the parameters of the second shell. This is  
the approach I find troubling. It seems to double count data. To be  
specific, the first shell fit might be from 1.0 to 1.5 angstroms, and  
the results would then be used to constrain the first shell in the  
next fit, which would vary second shell parameters but extend from 1.0  
to 2.0. The data from 1.0 to 1.5 has thus been used twice in a manner  
that would cause me some concern.

I am unclear as to which procedure Abhijeet is describing. I initially  
assumed the second, and Matt initially assumed the first; that was  
responsible for some of the confusion.

--Scott Calvin
Sarah Lawrence College



On Apr 18, 2009, at 1:57 PM, Matt Newville wrote:

> Hi Adam,
>
> I am confused by this thread too!
>
>
> I find the approach of "fit the first shell, then fix those parameters
> and fit the second shell" to be a reasonable way to start, so I
> responded.   I would not call this approach "acceptable, but generally
> not preferred" -- I would say it has actual merit.  Yes, it is often a
> good idea to check a fit to the whole spectrum toward the end of an
> analysis.  But in a fit to the whole spectrum, the quality of the fit
> to the more subtle features (the finer structure) can often get lost
> in the fit to the more dominant features of the spectra.  That is why
> we subtract mu0(E), after all.  I don't see Fourier filtering as a bad
> idea for analyzing higher shells.
>
> But perhaps I am just not understanding what Scott and Shelly  
> recommend???
>
> --Matt
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