[Ifeffit] High SO2

[Scott Calvin]

Sorry, I wasn't trying to be provocative or confusing.

I was following up on Abhijeet's original thread, which was about  
fitting a copper foil, presumably as practice for similar kinds of  
materials, such as alloys. I have seen many people in workshops  
proceed as he suggests with systems like crystalline metals: fit the  
first shell, take those results and prevent them from varying, expand  
the R-range and add paths and parameters, and then fit over the entire  
range but only varying the parameters for the outer path. I've never  
understood the benefits for that method with that kind of system. It  
doesn't mean there aren't any.

Then Jeremy responded that he used that approach, and, since I was  
still thinking of things like copper foils, I tried to confirm I  
understood the approach he was suggesting, and asked more about the  
reasoning.

Then you responded. It seemed to me you were discussing a very  
different kind of case from what I thought we were talking about, and  
in that case, it made sense to fix parameters from the first shell and  
then fit the second separately. So I tried to lay out some  
circumstances under which I would do that, as we now seemed to be  
talking about a broader set of cases.

So I'm not so much recommending anything right now as I am indicating  
what I do when and why I do it, and trying to figure out what other  
people are doing and why.

One possibility is that there are people who are frequently working  
with systems like the one you describe, and so they teach their  
students to use the approach where they fix the values for the first  
shell before moving on to the second as a general technique. The  
students might then apply that technique to the generic practice  
problem of copper foil, just to get used to how the approach, which  
will be quite appropriate for systems they will be working with.  
Because the majority of the cases I work with aren't like that, it's  
not what I teach my students to do, with the corresponding downside  
that it wouldn't be the first thing that would pop into their head in  
a system for which it was appropriate.

As we both know, fitting can be a complicated and somewhat subjective  
process. Both for my own skills and for my ability to teach others the  
technique, I would like to understand more about why people use the  
approaches they do.

--Scott Calvin
Sarah Lawrence College

On Apr 10, 2009, at 3:03 PM, Matt Newville wrote:

> Hi Scott,
>
> Sorry, I'm a little bit confused about what you are recommending for
> and against.   Earlier today you said
>   Well, I do know that many people do it that way, so it's not
> "wrong." But there are a number of problems with it:
>
> and
>   By doing that [not fitting the 1st shell with the 2nd], you're then
> distorting the fit to the next shell.
>
> and
>   Maybe someone who uses this strategy should speak up for it; I'd
> like to understand what the advantages are.
>
> When Jeremy responded, you asked him again what was the advantage of
> fixing the first shell while fitting the second shell.
>
> I read this (past and present form of "read") to mean that you do not
> approve of this approach. I do not see qualifications about what sorts
> of spectra you apply these rules to or mentions of special cases  But
> you were eager to hear other opinions because clearly many people use
> this "problematic" approach......
>
> Now you are saying that you sometimes do fit the 1st shell separately
> from the 2nd.   Strange.    It seems like you may have been
> deliberately provocative in order to get a response, or perhaps there
> is something else you are trying to get at?
>
> Could you please explain what you are recommending?
>
> --Matt
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