.. Athena document is copyright 2016 Bruce Ravel and released under The Creative Commons Attribution-ShareAlike License http://creativecommons.org/licenses/by-sa/3.0/ Computng potentials =================== Continuing with the example from the previous section, we can use an instrumented Feff object to compute potentials. .. code-block:: perl :linenos: #!/usr/bin/perl use Demeter; my $feff = Demeter::Feff -> new(file => "feff/feff.inp"); $feff -> set(workspace => "feff/", screen => 0,); $feff -> potph When the ``potph`` method is called, :demeter:`demeter` writes a :file:`feff.inp` file using the data from the original input file but with the ``CONTROL`` keyword set such that only the *potph* part of :demeter:`feff` gets run. At the end of this, :demeter:`feff`'s :file:`phase.bin` file will be written into the ``workspace`` directory. That's it. This part of :demeter:`feff` is used as-is by :demeter:`demeter`. In the course of fitting, you might find that you need to move an atom by such a large amount that you will want to recompute the potentials. For smaller moves, :demeter:`demeter` (and :demeter:`ifeffit`) assume that the primary effect of the move on the EXAFS is from changing the value of R in the EXAFS equation. Thus we assume that the changes in the scattering amplitude and phase shift due to the small change in the potential surface caused by a readjustment of the interatomic distance are small compared to effect of R in the EXAFS equation. At this time, there is not an obvious mechanism in :demeter:`demeter` to close this loop in the situation where the potentials need to be recalculated. That is, there are no tools for rewriting the atoms list in :file:`feff.inp` based on changes in inter-atomic distance uncovered in a fit. .. todo:: Track geometric parameters from :demeter:`atoms` to every site in the :demeter:`feff` input file. Then provide tools for parameterizing interatomic distances based on the geometry.