[Ifeffit] problem with E0 (enot) parameters

Scott Calvin SCalvin at slc.edu
Fri Jun 19 06:45:59 CDT 2009


Hi Darek,

You've got a false fit. The E0's aren't the biggest problem; look at  
the delr for potassium! Your fit is scrambling all the paths in non- 
physical ways.

Your initial description of the system suggests that you have a decent  
guess as to the structure to start with. What happens when you run a  
fit with very few free parameters? (Perhaps none, or perhaps just  
floating an overall S02, E0, and maybe a couple of sigma2's.) Does it  
look qualitatively right, with peaks where there should be peaks? If  
so, then you probably need a tighter set of constraints. If not, then  
the material is not what you think it is.

--Scott Calvin
Sarah Lawrence College

On Jun 19, 2009, at 7:30 AM, Zajac, Dariusz A. wrote:

> Hi Calvin,
> thanks for the email. I have tried to find something on mailing list
> about E0 but it seems that my "searching words" were not correct. I
> found only few posts but non of them explained (or followed) the  
> problem
> with large enot.
> enot parameters are closed to eachother e.g. enot_C = 12.597(+-0.654)
> and enot_N=12.407(+-1.776); correlations e.g. enot_N and delr_N=0.744,
> enot_C and delr_C=0.605 enot_C and enot_N=-0.313
> more detail you can find in the attachment or below. I have found that
> the value of the first background variable is large (-3437(+-23034))
> when I constrain both enot's parameters I do not see huge changes in
> parameters, chi^2 enot, amp, delr and ssh are the same in the range of
> uncertainties.
> I agree with you that program can shift not the correct parameter, but
> why every time I change parameters (I shift parameters from the local
> minimum) their come back?
>
> cheers
> darek
>
>
> Independent points          =      45.279296875
> Number of variables         =      30.000000000
> Chi-square                  =    219223.767
> Reduced Chi-square          =   14347.765383235
> R-factor                    =       0.022707172
> Measurement uncertainty (k) =       0.000102780
> Measurement uncertainty (R) =       0.000306293
> Number of data sets         =       1.000000000
>
>
> Guess parameters +/- uncertainties  (initial guess):
>  amp             =     0.8496050   +/-      0.0767160    (guessed as
> 0.849812 (0.095158))
>  enot_C          =    12.5967170   +/-      0.6539780    (guessed as
> 12.601228 (0.744286))
>  delr_C          =    -0.0055260   +/-      0.0068920    (guessed as
> -0.005265 (0.007572))
>  ss_C            =     0.0016740   +/-      0.0008460    (guessed as
> 0.001676 (0.000991))
>  enot_N          =    12.4065290   +/-      1.7757010    (guessed as
> 12.895967 (2.018563))
>  delr_N          =     0.0358140   +/-      0.0193390    (guessed as
> 0.042607 (0.029913))
>  ss_N            =     0.0068930   +/-      0.0016280    (guessed as
> 0.008196 (0.002599))
>  enot_K          =     0.0064050   +/-      9.7412920    (guessed as
> -4.066616 (13.931541))
>  delr_K          =     0.9265430   +/-      0.1815320    (guessed as
> -0.018996 (0.154984))
>  ss_K            =     0.0074030   +/-      0.0124260    (guessed as
> 0.011457 (0.010167))
>  enot_O          =    -8.4988040   +/-     13.7627610    (guessed as
> 0.906319 (10.059312))
>  delr_O          =    -0.3610320   +/-      0.0975640    (guessed as
> -0.139554 (0.069016))
>  ss_O            =     0.0010060   +/-      0.0050660    (guessed as
> 0.001140 (0.004395))
>
> Def parameters (using "FEFF0: Path 1: [C5_1]"):
>  enot_CN         =    12.5016230
>  delr_CN         =     0.0151440
>  ss_CN           =     0.0042840
>  enot_CNC        =    12.5333210
>  delr_CNC        =     0.0082540
>  ss_CNC          =     0.0034140
>  enot_NCN        =    12.4699250
>  delr_NCN        =     0.0220340
>  ss_NCN          =     0.0051540
>  enot_KN         =     6.2064670
>  delr_KN         =     0.4811790
>  ss_KN           =     0.0071480
>  enot_KC         =     6.3015610
>  delr_KC         =     0.4605080
>  ss_KC           =     0.0045380
>  enot_KNC        =     8.3365500
>  delr_KNC        =     0.3189440
>  ss_KNC          =     0.0053230
>
> Set parameters:
>  enot_H          =  -0.920939 (0.000000)
>  delr_H          =  0.103335 (0.000000)
>  ss_H            =  0.00749823 (0.000000)
>
> Background parameters +/- uncertainties:
>  bkg01_01        =  -3437.6705554   +/-   23034.7299514
>  bkg01_02        =    -0.0609379   +/-      1.8005458
>  bkg01_03        =     0.1948870   +/-      0.1595573
>  bkg01_04        =    -0.0398830   +/-      0.0358870
>  bkg01_05        =    -0.0025572   +/-      0.0147361
>  bkg01_06        =     0.0067564   +/-      0.0081250
>  bkg01_07        =    -0.0031386   +/-      0.0052991
>  bkg01_08        =     0.0006703   +/-      0.0037900
>  bkg01_09        =    -0.0000756   +/-      0.0028462
>  bkg01_10        =     0.0003428   +/-      0.0022025
>  bkg01_11        =    -0.0006552   +/-      0.0017628
>  bkg01_12        =     0.0006924   +/-      0.0015077
>  bkg01_13        =    -0.0004210   +/-      0.0015140
>  bkg01_14        =    -0.0002224   +/-      0.0022632
>  bkg01_15        =     0.0026167   +/-      0.0075534
>  bkg01_16        =    -0.0207863   +/-      0.0762094
>  bkg01_17        =    48.5761922   +/-   4197.5368888
>
>
> Correlations between variables:
>      enot_K and delr_K     -->  0.8386
>      enot_O and delr_O     -->  0.8120
>      enot_N and delr_N     -->  0.7440
>         amp and ss_C       -->  0.7232
>      enot_C and delr_C     -->  0.6048
>         amp and enot_N     -->  0.5755
>      enot_C and bkg01_02   --> -0.4957
>         amp and bkg01_05   -->  0.4817
>         amp and bkg01_04   --> -0.4376
>      enot_C and bkg01_03   -->  0.4175
>      enot_N and enot_O     -->  0.3976
>        ss_C and enot_N     -->  0.3902
>         amp and bkg01_06   --> -0.3779
>         amp and ss_N       -->  0.3767
>         amp and enot_C     --> -0.3652
>        ss_C and delr_N     -->  0.3422
>        ss_N and bkg01_04   --> -0.3330
>         amp and delr_N     -->  0.3325
>        ss_C and bkg01_15   --> -0.3311
>        ss_N and bkg01_05   -->  0.3172
>      enot_C and enot_N     --> -0.3133
>      enot_N and bkg01_02   -->  0.2957
>      enot_N and delr_O     -->  0.2918
>      delr_C and ss_N       --> -0.2832
>      delr_N and enot_O     -->  0.2804
>        ss_C and bkg01_16   -->  0.2690
>        ss_C and bkg01_05   -->  0.2663
>      delr_C and delr_N     --> -0.2572
>        ss_C and bkg01_14   -->  0.2569
>         amp and enot_O     -->  0.2560
>        ss_C and bkg01_04   --> -0.2528
> All other correlations are below 0.25
>
> Background parameters "bkg01_XX" belong to data set RP314a  
> K4W(CN)8x2H2O
> W:L3
>> -----Original Message-----
>> From: ifeffit-bounces at millenia.cars.aps.anl.gov
>> [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf
>> Of Scott Calvin
>> Sent: Friday, June 19, 2009 12:23 PM
>> To: XAFS Analysis using Ifeffit
>> Subject: Re: [Ifeffit] problem with E0 (enot) parameters
>>
>>
>> Hi Zajac,
>>
>> What happens if you constrain all E0's to be the same? In the fit
>> where they come out large, what are the uncertainties in the E0's?
>> What are their correlations with other fitted parameters? There has
>> been some debate in this list on the past as to how useful it is to
>> allow for different E0's for different paths. It may be that Artemis
>> is shifting the E0's for those paths in lieu of some other correlated
>> parameter.
>>
>> --Scott Calvin
>> Sarah Lawrence College
>>
>> On Jun 19, 2009, at 5:33 AM, Zajac, Dariusz A. wrote:
>>
>>> Hi all,
>>> can anybody help me and send some link about problems with enot in
>>> Artemis? in google, tutorials I couldn't find any help. Of
>> course are
>>> few posts about delr or ss parameters, but enot is somehow
>> omited (or
>>> I can not find it...)
>>>
>>> the problem is with fiting K4W(CN)8*2H2O at W:L3 edge.
>>> fit and others parameters looks ok, except enot for C and N, where
>>> both
>>> are around 12 and don't want to fit to other values...
>>> thanks
>>> darek




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