Dear XAS Community, I stumbled over the issue that a PCA on 20 EXAFS spectra (k², k = 2.0-11.0 Å-1) perfomed in Sixpack does not give the same results (Eigenvalues, variance) as in Athena. However, when I use other statistical programs (i.e., TIBCO Statistica or SPSS), I get the same results as reported in Athena. I tested this with another EXAFS dataset of over 30 samples and the problem persited. An old entry from 2017 in the ifeffit mailing list ("[Ifeffit] Calculation of SPOIL value for the reconstruction of standard spectra"), told me that as of Sixpack version 1.4 on, a new/different PCA algorithm from the scikit-learn Python package is used. So I downloaded older versions of Sixpack (i.e. 1.3) and used "Use Old PCA"-selection in the "Rotation" menu bar, which actually gave different results. However, they are still different from the Athena/Statistical program results. My question is: What is behind this? Is there some sort of normalization or axis rotation, that leads to the different values? Is there any way to change this so that the results are comparable to other programs? As I need to use the Target Transform option after PCA, which is not yet possible in Athena, I am at a loss as to how to deal with these different results and where they come from. Thank you very much for your help, Teresa -- Teresa Zahoransky Soil Mineralogy Gottfried Wilhelm Leibniz Universität Hannover Institute of Mineralogy Callinstr. 3, Room 325 D-30167 Hannover, Germany Phone: +49 (0)511 762-8058 Email: t.zahoransky@mineralogie.uni-hannover.de
