.. Artemis document is copyright 2016 Bruce Ravel and released under The Creative Commons Attribution-ShareAlike License http://creativecommons.org/licenses/by-sa/3.0/ Aggregate Feff calculation ========================== If crystal data has two or more inequivalent sites occupied by the same kind of atom, some additional functionality is enabled when one of the inequivalent sites is selected as the absorber. As you can see in the following screenshot, the :button:`Aggregate,light` button is available, as are controls for setting the `fuzzy degeneracy <../extended/fuzzy.html>`__ parameters. .. _fig-feffzirconolite: .. figure:: ../../_images/feff-zirconolite.png :target: ../_images/feff-zirconolite.png :align: center Crystal data for CaZrTi\ :sub:`2`\ O\ :sub:`7` with a Ti atom selected, enabling the aggregate calculation controls. In the aggregate :demeter:`feff` calculation, the path finder is run for each inequivalent position containing the same central atom. The path lists are merged together, weighted by fractional population of the site in the unit cell, before running the check for `fuzzy degeneracy <../extended/fuzzy.html>`__. The weighting by population fraction means that a bin of paths can be occupied by a non-integer number of atoms. In the example of CaZrTi\ :sub:`2`\ O\ :sub:`7`, there are three Ti sites. The are 8 :guilabel:`Ti1` atoms in the unit cell and 4 each of :guilabel:`Ti2` and :guilabel:`Ti3`. Thus half the Ti atoms are from site 1 and a quarter each from sites 2 and 3. Each Ti site is surrounded by 6 oxygen atoms at a variety of distances. :guilabel:`Ti1` has O atoms at 1.843 |AA|, 1.880 |AA|, 1.927 |AA|, 1.987 |AA|, 2.0007 |AA|, and 2.023 |AA|. :guilabel:`Ti2` has 2 O atoms at each of 1.786 |AA|, 2.050 |AA|, 2.498 |AA|. :guilabel:`Ti3` has 2 O atoms at each of 1.975 |AA|, 1.877 |AA|, 1.975 |AA|. In short, the Ti K edge of CaZrTi\ :sub:`2`\ O\ :sub:`7` is a mess! Using a bin size of 0.1 |AA| and combining the three sites together weighted by their fractional populations in the unit cell, we end up with 4 distances. There are 2.5 O atoms at 1.852 |AA|, 2.5 O atoms at 1.984 |AA|, 0.5 O atoms at 2.050 |AA|, and 0.5 O atoms at 2.498 |AA|. Instead of having to parameterize 12 different Ti-O distances, keeping track of fractional populations when parameterizing the S\ :sup:`2`\ :sub:`0` values for each path, the aggregate :demeter:`feff` calculation requires managing only 4 paths. For a complete discussion of the the aggregate :demeter:`feff` calculation, see .. bibliography:: ../artemis.bib :filter: author % "Ravel" and year == '2014' :list: bullet