[Ifeffit] Artemis and templates

Bruce Ravel ravel at phys.washington.edu
Tue Oct 22 15:45:09 CDT 2002

Hi folks,

I received this question today:

  > The third question is about the fitting templates provided in your
  > software, such as firstshell, mixedshell, three shell, etc. How
  > can I predict which one is good for the fitting? What are the
  > differences among them?

A bit of recent history.  The templates are a feature of Artemis that
I started working on just prior to last month's EXAFS course at NSLS.
The concept seemed to be working pretty well for copper metal
($ironic_mode=1), so I decided to put the templates in the official

The thing that the templates have been most useful for so far is to
point out that Artemis remains almost completely undocumented.  The
templates are but one confusing part of the program.  Once a user gets
confused, there is no place (other than, of course, this mailing list)
for the user to turn for information.  Sigh.

Let me start by explaining what the templates are not:

They are not intended to be a complete description of any fit to any
data set.  That is, you cannot expect to choose an item from the
Templates menu, click the big green fit button, and be ready to

The templates are intended to help you start a new fitting project by
filling in many of the little entry boxes scattered throughout the
program with reasonable first guesses for interesting parameters.
Another way to say this is that the templates will hopefully help save
you from having to do a certain amount of really tedious typing.

As an example, the template called "firstshell" makes a non-stupid
guess about what kinds of parameters you might want for a fit to the
first shell of your data.  You will almost certainly have to edit the
guess/set/def parameters as well as the path parameters to get a fit
to your data that is statistically valid and physically reasonable.

Some of the templates ("threeshell" for example) do nothing at all at
this time.

So, to answer the questions directly:

  > How can I predict which one is good for the fitting?

In a sense, none of them are.  However, if you want to do a fit to the
first shell, the "firstshell" template may be helpful.  If you want to
do a fit for which the correlated Debye model is valid, the "debye"
template may be helpful.

  > What are the differences among them?

Their names tend to explain the kind of situation in which they might
be helpful.

Let me end by saying my main point one more time in different

  Templates are the *starting points* for fits.  Use them in that
  sense.  They are NOT complete fitting models.  Do not use them as

Hope that helps,

 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
 Code 6134, Building 3, Room 222
 Naval Research Laboratory                          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b, X24c, U4b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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