Ok, that makes things much more clear.
Thankfully my group has the book, ill be taking a look at that.
Thank you for the quick and clear responce.

Much Obliged 

Mikhail Solovyev

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Today's Topics:

   1. delR and sigma2 models for multiple Scattering Paths
      (Mikhail Solovyev)
   2. Re: delR and sigma2 models for multiple Scattering Paths
      (Bruce Ravel)


----------------------------------------------------------------------

Message: 1
Date: Wed, 18 Jan 2017 13:27:29 -0500
From: Mikhail Solovyev <sarsol89@gmail.com>
To: ifeffit@millenia.cars.aps.anl.gov
Subject: [Ifeffit] delR and sigma2 models for multiple Scattering
        Paths
Message-ID:
        <CABVdVPzfO9KQ_oSkki4cN3RspBfD9+2Gh7Bf0wdS5soT1-J_jw@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

Hello to all.
I'm currently trying to Artemis fit a molecule that is highly symmetric
with multiple scattering paths playing a large role.
delR and sigma2 have been handled rather well for all of the single paths.
I am, however, confused as to how to handle it for multiple scattering
paths.
Reff for them seems to be close to the value of the furthest atom, but is
always ~5-10% off. (which would be my second question, how is Reff
determined for multiple scattering paths)
I've looked at the presentation and examples, such as FeS2.
I am trying to incorporate all of the same variables used in single
scattering paths for multiple scattering paths the best i can.

Thank you ahead of time.

Mikhail Solovyev
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Message: 2
Date: Wed, 18 Jan 2017 13:47:19 -0500
From: Bruce Ravel <bravel@bnl.gov>
To: XAFS Analysis using Ifeffit <ifeffit@millenia.cars.aps.anl.gov>
Subject: Re: [Ifeffit] delR and sigma2 models for multiple Scattering
        Paths
Message-ID: <89ff75ad-447d-6e47-26b9-7a774c10449f@bnl.gov>
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On 01/18/2017 01:27 PM, Mikhail Solovyev wrote:
> I'm currently trying to Artemis fit a molecule that is highly symmetric
> with multiple scattering paths playing a large role.
> delR and sigma2 have been handled rather well for all of the single paths.
> I am, however, confused as to how to handle it for multiple scattering
> paths.
> Reff for them seems to be close to the value of the furthest atom, but
> is always ~5-10% off. (which would be my second question, how is Reff
> determined for multiple scattering paths)
> I've looked at the presentation and examples, such as FeS2.
> I am trying to incorporate all of the same variables used in single
> scattering paths for multiple scattering paths the best i can.

Mikhail,

The definition of Reff is /always/ "half-path-length".

For a single scattering path, this is trivial.  It's simply the
inter-atomic distance.

For a multiple scattering path, you add up the length of each leg, then
divide by 2.  For the case of strictly collinear multiple scattering
path, Reff is the distance between the two atoms farthest apart.
However, if there are scattering angle that are something other than 0
or 180 degrees, Reff will be related to interatomic distances via some
kind of trigonometric relationship that has to do with scattering angles.

You are wise to try to build constraints between SS and MS paths in your
fit.  For DeltaR parameters, trigonometry helps, but may only be an
approximation.  Possibly a good approximation, but an approximation
nonetheless.


Scott discusses how to parameterize sigma^2 in his book:

https://www.amazon.com/XAFS-Everyone-Scott-Calvin/dp/1439878633

There are some situations (see https://doi.org/10.1103/PhysRevB.54.156)
where MS sigma^2 can be related exactly to SS.  In general, though, MS
sigma^2s usually can only be approximated by constraint with the SS
sigma^2s.

https://speakerdeck.com/bruceravel/discussion-of-the-fes2-exafs-analysis-example?slide=6

HTH,
B


--
  Bruce Ravel  ------------------------------------ bravel@bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Building 743, Room 114
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
  Software:    https://github.com/bruceravel
  Demeter:     http://bruceravel.github.io/demeter/


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