Dear Patricia,
Linear combination fits are a nice tool, but one should handle them with great care - the background subtraction process can introduce significant errors. You have to fine tune the parameters by hand if you are using Athena to get reasonable results - sometimes PCA (principle component analysis) helps to find a reasonable number of components - Athena only shows the fitting errors: please keep in mind, that the error of the complete measurement and data evaluation process is much larger. It makes sense to test the process with known mixtures. - the variation of the of the sum of all components between 98 and 105 % (even with choosing -> sum coefficients to 1) is a problem somewhere inside of Athena. I think only Bruce Ravel can help you there. best regards Stefan > Am 08.03.2021 um 19:27 schrieb Patricia Poths <patriciapo...@chem.ucla.edu>: > > Hi, > > I am a theoretical chemistry PhD student working on fitting experimental > spectra with computed spectra in order to get a better understanding of the > composition. In the LCF process with athena, I have found that when I allow > "sum coefficients to 1", I get an unphysical negative coefficient of the last > standard- after reading through the mailing list I understood why, and so no > longer use that. The sum of my coefficients during the fitting now is close > to 1- generally within the range of 0.95-1.1 at the absolute extremes, but > more often around ~0.98- ~1.05. > > In order to compare these coefficients, I renormalize them to 1, so they can > represent the fractions of each component present. However, to test this I > took the new normalized coefficients and summed up the standards with their > respective weights to create this normalized "fit", and found that it is > worse than the initial one. Is this something I should be concerned about > when reporting the qualitative trends in how the composition changes? And if > so, is there a better way to do the fits in a more normalized way? > > Many thanks, > Patricia > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit
