Hello All, can someone please help with this: (1) feff requires that the adsorbing atom be "embedded" (that is, surrounded by) other atoms in order to calculate potentials correctly, but how deeply does the atom need to be embedded? For example, if I construct a model of an adsorbate on a mineral surface, where that adsorbate has a full complement of 1st shell coordinating OH, O, and H2O groups as appropriate, but if I do not include all the uncoordinated f water atoms that would be nearby (so that I essentially have a surface in vacuum), will the potentials for my central atom be calculated correctly (or correctly enough for use in IFEFFIT)? a more specific example: the AsO4 tetrahedron bonded to the goethite surface, which is made of octahedrally-coordinated iron. The arsenate bonds to the goethite by sharing 1 edge of the tetrahedron with the apices of 2 adjacent Fe octahedra. a related question that comes to mind: if I just wanted to see the signal from the 2 adjacent Fe atoms, could I create a perfectly "useable" FEFF model (for IFEFFIT) by limiting my calculations to the small cluster of 1 arsenate tetrahedron and 2 Fe octahedra? thanks, andrea ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Andrea L. Foster, Ph.D. U.S.G.S. WMESC 345 Middlfield Rd., MS 901 Menlo Park, CA 94025 T: 650-329-5437 F: 650-329-5491