[Ifeffit] Imidazole multiple scattering in Artemis

[Matthew Newville]

Hi Chris,

Using the single-scattering and multiple-scattering paths that have the same scattering atoms is not double counting. In general, including all the paths from a Feff calculation is what you ought to do to account for all of the EXAFS.  But many paths will have weak contributions, and so can often be neglected.

It gets more tricky to avoid double-counting if you mix paths from multiple Feff calculations, but it sounds like that is not what you're doing.

Cheers,

--Matt


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From: Ifeffit <ifeffit-bounces at millenia.cars.aps.anl.gov> on behalf of Chris Pollock <cjp227 at cornell.edu>
Sent: Tuesday, April 2, 2024 6:18 PM
To: XAFS Analysis using Ifeffit <Ifeffit at millenia.cars.aps.anl.gov>
Subject: [Ifeffit] Imidazole multiple scattering in Artemis

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Hi everyone,
I've been using Artremis for quite a while, but have only recently come across a case where the inclusion of multiple scattering seems to be important for fitting some data and wanted to ask what is probably a super basic question to make sure I don't mess anything up.

I've recently collected some data on a metalloenzyme with a quite histidine-rich active site and, since it's been shown previously that multiple scattering is important for fitting the longer range scattering for these types of systems, I wanted to make sure I capture this in my Artemis fits.  The data appear to show scattering contributions from N3, C2, and N1 in the imidazole ring, and FEFF seems to agree that those paths are the most important, though it calculates the MS intensity to be significantly greater than the single scattering for the farther N (N1).  So, my super naive question is:  To include multiple scattering paths in the fits--say, the Fe-N3-N1 triangle--is it correct to include the Fe-N3-N1 multiple scattering path in addition to the Fe-N3 and Fe-N1 single scattering paths, or would that effectively be double counting the farther nitrogen (N1)?

Thanks in advance!
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