Dear Panel of Feffit Experts, I have two questions for you, with several parts. I am looking at Lithium Cobalt Oxide annealed at 200 C. (1) In Athena what exactly do the varying levels of spine clamps mean? When do I need to use them? I've been ignoring them up to now, but I think that it might help my resulting fit, which brings me to my second part. (2) By looking at my results is there any advice you can give me? Do my reduced chi-squared, R-factor and correlated variables look good? I know that the R-Factor should be below 0.05, but what about red chi-squared (depends on the material)? Also, I am concerened that my amp (SO^2) should be closer to 1. I am using the einstien model and e01 refers to the farther 3 of my 4 paths. What else should I consider? Thanks, Dan Carter Hunter College Results: Independent points = 11.225585938 Number of variables = 5.000000000 Chi-square = 66.894398861 Reduced Chi-square = 10.745076774 R-factor = 0.040027549 Measurement uncertainty (k) = 0.002990761 Measurement uncertainty (R) = 0.069932022 Number of data sets = 1.000000000 Guess parameters +/- uncertainties: e0 = -3.9151859 +/- 2.3534572 delr = -0.0189516 +/- 0.0134287 ampg = 0.9492922 +/- 0.0001603 e01 = -9.3741702 +/- 2.5615506 theta = 415.2229263 +/- 89.8481275 Restraints: amp = 0.7077880 Set parameters: ampt = 0.9500000 scale = 1000.0000000 temp = 300.0000000 Correlations between variables: ampg and theta --> -0.9015 delr and e01 --> 0.8551 e0 and delr --> 0.7809 e0 and e01 --> 0.6858 All other correlations are below 0.25
Hi Dan, On Wed, 28 Jan 2004 dmc@pdx.edu wrote:
(1) In Athena what exactly do the varying levels of spine clamps mean?
They mean how strongly to try to make the background mu0(E) match the value of mu(E) at the endpoints. Athena uses the qualitative terms 'strong', 'rigid', and so on because it's difficult to give physical meaning to the weighting factors used for this portion of the fit.
When do I need to use them?
If the background is clearly diverging from mu(E) at high-k, a clamp is a useful thing to use. At low-k it's harder to tell what the background function is supposed to do, so it's even more of a judgement call of when to use the low-k clamp.
I've been ignoring them up to now, but I think that it might help my resulting fit, which brings me to my second part.
Hopefully, a spline clamp will not greatly alter fitting results. The main way the background removal _should_ affect fit results is how the normalization is done, and what the edge step gets set to.
(2) By looking at my results is there any advice you can give me? Do my reduced chi-squared, R-factor and correlated variables look good? I know that the R-Factor should be below 0.05, but what about red chi-squared (depends on the material)?
From what the limited information you posted, it looks OK. An R-factor > 0.05 often indicates a bad fit, but that value is not case in stone. The values of chi-square and reduced chi-square are complicated by depending on the estimated uncertainty in the data which is an estimate. Statistics 101 says that reduced chi-square should be ~1 for a 'good fit', but Statistics 101 assumes that a) you know the uncertainty in the data, b) you know how many independent measurements (ie, how much data) you have, and c) the model has no error in it. For EXAFS, these can all be questioned. Experience says that a reduced chi-squares around 10 are common for excellent fits to data of well-characterized standards, such as a metal foil or powdered metal oxide.
Also, I am concerened that my amp (SO^2) should be closer to 1.
Sorry to be so dense, but is S02 set as 'amp' (which is restrained somehow, and ending up at 0.707), 'ampg', or 'ampt'? The standard answer is that S02 between 0.7 and 1.0 is 'normal'. To say any more, you'd want to start checking the normalization and energy resolution. It might be worth thinking about how S02 is restrained (if it is), and why the uncertainty in ampg is so small. Hope that helps! --Matt
participants (2)
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dmc＠pdx.edu -
Matt Newville