[Ifeffit] Using parameters in Artemis

[Carlo Segre]

Hi David:

On Thu, 8 Mar 2007, David Weedon wrote:

>
> In the Journal entry we are asked to click "Guess, Def, Set", then click
> "new" and enter DelScale = 0.  After I hit the "Guess" button I cannot
> find the parameter "dr" on the list.  Am I correct in assuming that "dr"
> is some kind of reserved parameter name that should not be used in
> equations?  Which leads me to ask several questions (probably silly
> questions)???
>

older versions of Artemis used "dr" but it is a reserved name in the 
latest versions.  Use "delr" or something else instead.

>
> QUESTIONS:
>
> 1                     I presume all the parameters like "amp, deg, delr,
> ss2 and enot" relate to the obvious parameters in the EXAFS eqn.
> Assigning numerical values makes sense but what is going on when we
> create mathematical functions?? Is there a list of functions that we can
> model here? Is it somewhere in the software?
>

I presume that you mean like using the debye function to parametrize the 
sigma parameters?  The functions available are those in Ifeffit.  The best 
thing to do is to consult the Ifeffit reference manual (page 12) for a
list of these.

http://cars9.uchicago.edu/ifeffit/doc.html

>
>
> 2                     What are we doing when we make our own parameters?
> Are they a mixture of parameters from the EXAFS equation that we wish to
> evaluate in the fit, and if so why?
>

Sometimes, you wish to parametrize your structural model interms of 
specific parameters.  For example the lattice parameter of a unit cell. 
In order to do this, you need to relate the fundamental parameters of each 
EXAFS path to these more physically "meaningful" quantities.  I know this 
is sort of a vague answer but hopefully it helps.  There are some examples 
available in the horae terball.

Hope this helps,

Carlo

-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Special Projects, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu    http://www.iit.edu/~segre    segre at debian.org