[Ifeffit] The most important fitting parameters

[Kelly, Shelly D.]

Hi Bridget,

Here are some other things to consider. 
1)  Is the second oxygen shell really necessary?  What happens when you
leave it out of the fit and remove 3 parameters?  Whenever I "make-up" a
structure I always take the minimalist approach.  If the
reduced-chi-squares for the two different models are not more than ~50%
different then use the fewest paths possible.

2) Is the structure really the "same" as CuWO4?  What happens when you
use the same delr=alpha*reff for all 4 paths.  What happens when you use
CuWO4 numbers for the amplitudes?

3)  Is the structure made of smaller bits of molecules that you can
constrain in some way?  For example, maybe you know that the second O
atom is attached to either a neighboring Cu atom or the neighboring W
atom.  Then you could have the same delr for both the second O and the
other neighbor attached to it (assuming the angle of the molecule isn't
another variable). Similarly, for the amplitudes, maybe you know that
one additional W neighbor will always bring with it "two" O2 atoms, so
that you can constrain the amplitudes.

4)  Do you have data from CuWO4?  Or can you find it somewhere?  Maybe
the EXAFS model of that compound could be refined with your unknown and
some of the parameters could be constrained to the same values.

5)  Look carefully at the fit-values and the uncertainties.  If the
value +/- the uncertainties overlaps for two different
alphas/amps/sigmas you might consider containing the values to the same
variable.  You need to have some physical intuition that gives you
confidence in this type of constraint.


6)  Sometimes it isn't possible, but I always feel better about my
models if I have 2times the number of independent points as the number
of variables.  Always work hard to get the number of variables as low as
possible.


Well, that is all that comes to mind.
HTH,

Shelly
 

---------------------------
Shelly Kelly
Argonne National Laboratory
Bldg 203, RM E113
9700 S. Cass Ave
Argonne, IL 60439

Molecular Environmental Science Group
www.mesg.anl.gov
skelly at anl.gov
phone: 630-252-7376

-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Bridget
Ingham
Sent: Friday, June 16, 2006 8:27 AM
To: 'ifeffit at millenia.cars.aps.anl.gov'
Subject: [Ifeffit] The most important fitting parameters

Dear all,

I am trying to use EXAFS, along with other techniques, to attempt to
find
information about an unknown structure. It is an organic-inorganic
hybrid
material, EDX reveals Cu1-W1-O4 as the stoichiometry of the inorganic
component, which we are most interested in structurally. We have not
been
able to solve the structure from XRD, although not for want of trying.

I started off with CuWO4 as the model compound and constrained the
fitting
parameters:
- take all 1st oxygen shells (x6 at slightly different distances) and
give
them all amp1, dr1*reff, enot and ss1
- take all 1st copper shells (x2) and give them both amp2, dr2*reff,
enot
and ss2
- take all 2nd oxygen shells (x6), amp3, dr3*reff, enot, ss3
- take all 1st tungsten shells (x6), amp4, dr4*reff, enot, ss4

This gave me 13 parameters with 14.8 independent points.

(Other factors: fitted in kw=2&3, k-range 2-9, used Athena with default
background subtraction parameters as the chi(k) from that seemed
sensible.
The data were collected at SSRL, beamline 11-2, in transmission.)

I was happy with the final result (R-factor 0.014), but am just
wondering if
this is a sensible way to go about doing things - bearing in mind I am
not
sure of the structure in the first place. Any hints or advice would be
much
appreciated. I should have collected data at the tungsten edge as well,
as
it is not that much higher in energy from Cu, but this didn't occur to
me
until later.

Regards,
Bridget

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