[Ifeffit] Normalization of LCF coefficients when fitting experimental spectra with theoretical spectra

Patricia Poths patriciapoths at chem.ucla.edu
Mon Mar 8 12:27:39 CST 2021


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,
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