[Ifeffit] Artemis questions

Bruce Ravel ravel at phys.washington.edu
Fri May 16 09:15:36 CDT 2003

Hi Carlo,

Hope you don't mind my responding to the list.  These are questions
that I think will be of general interest.  I am actually going to
split my answers into two emails, both of which will still be rather
long -)
> I have been working with Artemis over the past few days and I have a
> couple of questions that I cannot seem to find answers to in the manuals
> for FEFFIT and Artemis.

OK!  You have me up against the wall ;-) Documentation is currently
the Artemis' great shortcoming.  I apologize that the doc is coming
along slowly.  FWIW, I have been thinking alot about the doc, but
haven't found a lot of time to write.
> 1. What is the difference between a SET and a DEF quantity.  I suspect
> that it is an issue of being updated or not but I am not sure.

A SET parameter is evaluated at the start of a fit and remains
invarient throughout the fit.

A DEF parameter is re-evaluated at every iteration of the fit.
Typically a def parameter depends on one or more guess parameters.

Let's consider a simple example.  Suppose that we have a crystal with
a nominal lattice constant "a0" and we want to consider the
possibility in the fit that the actuall lattice constant deviates from
"a0".  To do so, we may consider a parameter "delta_a":

     set   a0       3.61
     guess delta_a  0.0

Now suppose we want to use "a" (the measured lattice constant) in math
expressions for some of our path parameters.  Do we say

     set   a        a0 + delta_a
     def   a        a0 + delta_a
The answer is the second one.  If make "a" a SET parameter, then a
will be fixed to 3.61 at the beginning of the fit and not change.  If,
however, we make "a" a DEF parameter, then it will be updated every
time that "delta_a" is updated during the fit.

At the risk of confusing the issue, I feel I should make a connection
with Matt's old feffit program.  In the old feffit, set parameters
behaved like Ifeffit's def parameters.  That is, in the old feffit,
set parameters were updated throughout the fit.

As a rule of thumb for Artemis, if your parameter is a number, make it
a set parameter.  If it is a math expression, make it a def parameter.


 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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