[Ifeffit] How to keep enot, delr, and ss within acceptable limits

Scott Calvin SCalvin at slc.edu
Fri Jan 9 11:46:22 CST 2009

Hi Hashem,

Yes, there is, sort of.

Use a restraint of this form: Penalty(enot,-10,10)*1000

This adds a penalty to chi-square when enot is outside of the range  
-10 to 10. The *1000 part just makes it a very steep penalty; you can  
make it smaller (or larger) to allow it to drift a little outside the  
targeted range.

Having said that, I find that it usually doesn't work as desired. Most  
of the time the fit just goes to the edge of wherever you set the  
penalty and sits there. That means there's an underlying problem in  
the model or the data which really needs to be addressed. Once in a  
while, though, the penalty function will keep you out of a false  
minimum and put you in the right one.

--Scott Calvin
Sarah Lawrence College

On Jan 9, 2009, at 12:34 PM, Hashem Stietiya wrote:

> Dear All,
> I am currently using ARTEMIS to fit Zn-O, Zn-Zn paths to first and  
> second shells, respectively. Many times I would get unacceptable  
> values for enot, ss, and delr. My question is: is there a way to  
> tell ARTEMIS that I want these parameters to be within acceptable  
> limits? For example, you cannot have negative ss, so how can I avoid  
> any fit result that produces negative values for ss? Or high  
> extremely high values for enot and delr?
> I appreciate your assistance and guidance.
> Regards,
> Hashem Stietiya
> School of plant, Environmental and Soil Sciences
> Louisiana State University

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