[Ifeffit] Questions for Artemis fit

[huyanyun at physics.utoronto.ca]

Hi all,

How would you deal with the case that the absorber atoms are occupying  
two different crystallography sites? Should I still use/guess the same  
'S02' for paths calculated from two sites?

Best,
Yanyun


Quoting huyanyun at physics.utoronto.ca:

> Hi Scott,
>
> Thank you for giving me detailed examples. I will go with your  
> suggestion and try to reduce free parameters.
>
> Best,
> Yanyun
> Quoting Scott Calvin <scalvin at sarahlawrence.edu>:
>
>> Hi Yanyun,
>>
>> On Aug 19, 2014, at 12:05 PM, huyanyun at physics.utoronto.ca wrote:
>>>
>>> But my question is, for each path there are a set of Path Parameters,
>>> does that mean we have to fit this set of path parameters independent
>>> from those used in other paths?  For instance, if I am going to
>>> include 30 paths in my fitting, do I need to guess about 150
>>> (30*5=150) parameters (except guess the same 'enot' for all paths)?
>>
>> No. One of the great principles of Ifeffit (and thus Artemis) is  
>> that the path parameters do not have to be the same as the fit  
>> parameters.
>>
>> As a simple example, it's good to start with the model that all  
>> paths have the same value for E0. So you can guess a single  
>> parameter and then use that parameter for the E0 for all paths.
>>
>> As another simple example, a cubic crystal might be modelled as  
>> having a uniform thermal (or Vegard's law) expansion. Then there  
>> could be a single guessed parameter indicating the fractional  
>> expansion (called, for example, alpha) while the delr for each path  
>> could then be entered as alpha*reff.
>>
>> Much more complicated constraint schemes are possible, but for fits  
>> with large number of paths, the number of guessed parameters is  
>> almost always much much lower than the number of path parameters.
>>
>>>
>>> As shown on the fit Log file, the Correlation value between two fit
>>> parameters is the bigger the better, or the smaller the better?
>>
>> Not necessarily. Correlations are provided to help understand the  
>> relationship between guessed parameters, but there's nothing  
>> inherently wrong with a high correlation...the uncertainty  
>> associated with that relationship is already represented in the  
>> uncertainties reported with the fit. In other words, it is more  
>> direct to focus on getting the uncertainties to be lower, rather  
>> than the correlations.
>>
>> --Scott Calvin
>> Sarah Lawrence College
>>
>>
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>
>
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