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-exa... 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/