Hi Dan, Why are you using only one delr value and two dE values? I would suggest that you try it the other way around. two or more delr values with only one dE value. When you try the different models compare the reduced-chi-square values as you are doing but also look at the Backfourier transfrom of the data and the fit - or the chi(k) data if it is clean. Are they crossing zero at the same places. Do they miss more at low q or high q? Missing at low q/k would suggest separate dE values while missing at high q/k would suggest separate delr values. Would your model be better described by using an overall expansion/contraction term rather than one delr value? Using one delr means that all the paths move by -0.01 angstroms. Using one expansion term would allow all paths to contract by 0.1% which might be more reasonable. So that the long paths shrink in more than the short paths. To use a expansion term use the special "reff" variable which stands for the "effective r" i.e. the effective half path length. set delr1 = alpha*reff set delr2 = alpha*reff guess alpha= -0.01 -yes please drop the third cummulant. HTH Shelly
-----Original Message----- From: dmc@pdx.edu [mailto:dmc@pdx.edu] Sent: Tuesday, August 17, 2004 1:24 PM To: XAFS Analysis using Ifeffit Subject: [Ifeffit] MoS2 fit
Hi Mailing list,
Well, the chief concern I have with my fit now is that I have some negative parameters (delr and e0_1):
Guess parameters +/- uncertainties (initial guess):
amp = 0.7709790 +/- 0.0631820 e0 = 1.1165150 +/- 1.0100360 delr = -0.0106440 +/- 0.0077110 ss = 0.0032510 +/- 0.0005200 e0_1 = -4.6848070 +/- 1.7781210 third = 0.0000370 +/- 0.0001070
So, I guess I can drop the third cumulant.
This is a fit using 6 paths (1,2,3,6 are single scatterers) These are the results this time around:
Reduced Chi-square = 410.285252396 R-factor = 0.014599348
As you can see I am still using two e0's, it is still dramatically improving the fit. Without: Reduced Chi-square = 1133.485950636 R-factor = 0.044551155
any advice welcomed, Dan
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Hi Shelly, Thanks for the suggestions, here's what happened.
Backfourier transform: Do they miss more at low q or high q? Missing at low q/k would suggest separate dE values while missing at high q/k would suggest separate delr values.
It looks to me like they are missing at low q/k. I will try to attach a graph.
Would your model be better described by using an overall expansion/contraction term rather than one delr value? Using one delr means that all the paths move by -0.01 angstroms. Using one expansion term would allow all paths to contract by 0.1% which might be more reasonable. So that the long paths shrink in more than the short paths. To use a expansion term use the special "reff" variable which stands for the "effective r" i.e. the effective half path length.
I did this and it did help my reduced chi, but I only needed one delr. It didnt seem to make a difference having two. Also, don't forget you have to "define" when using equations, I got an error message when I used "set." Current Results: Chi-square = 4121.564440839 Reduced Chi-square = 390.207284340 Guess parameters +/- uncertainties (initial guess): amp = 0.7738890 +/- 0.0616540 e0 = 1.2375470 +/- 0.8909140 ss = 0.0032920 +/- 0.0005050 e0_1 = -5.3762680 +/- 1.4964040 alpha = -0.0044720 +/- 0.0019360 (-0.0100) There are still some negative values, I guess thats ok? Mucho Thanks , Dan
On Tuesday 17 August 2004 03:02 pm, dmc@pdx.edu wrote:
Also, don't forget you have to "define" when using equations, I got an error message when I used "set."
Which error message did you get? Since last week's business about setting the feff executable correctly, I have started examining artemis's error/warning/informational messages and changing some that seemed vague to me. I also wanted to point out the difference between set and def in Ifeffit, which may help to explain whatever the message was. In Ifeffit, a set parameter is evaluated once when the command is issued to Ifeffit. A def parameter is parsed and it's math expression is stored. As the fit progresses, the math expression is re-evaluated to be kept up to date with the current values of the guess parameters. A set, on the other hand, does not change during the course of the fit. HTH, B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b 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/
It just gives me the old box with : "there are errors in your math expression.
Do you wish to abort or carry on..."
I clicked on "abort," then defined the parameter and it worked fine.
--Dan
Quoting Bruce Ravel
On Tuesday 17 August 2004 03:02 pm, dmc@pdx.edu wrote:
Also, don't forget you have to "define" when using equations, I got an error message when I used "set."
Which error message did you get? Since last week's business about setting the feff executable correctly, I have started examining artemis's error/warning/informational messages and changing some that seemed vague to me.
I also wanted to point out the difference between set and def in Ifeffit, which may help to explain whatever the message was.
In Ifeffit, a set parameter is evaluated once when the command is issued to Ifeffit. A def parameter is parsed and it's math expression is stored. As the fit progresses, the math expression is re-evaluated to be kept up to date with the current values of the guess parameters. A set, on the other hand, does not change during the course of the fit.
HTH, B
-- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642
NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b 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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participants (3)
-
Bruce Ravel -
dmc＠pdx.edu -
Kelly, Shelly D.