Hi folks, I suppose I should preface this message by saying that I will not say anything new and profound. Indeed, this email is something of a rant -- so some of you may want to click ahead to the next email right now and avoid the whole darn thing! Today I was asked a Frequently Asked Question about using atoms and feff and getting started with some fits. This particular question is pretty common, but it's one that kind of bugs me. It's been rattling around in my brain for the remainder of the evening. To get it out of my brain, I figured I would write a message to the mailing list. A colleague showed me some data and explained a bit about the sample. This person had the lattice constants and space group for the oxide form of the absorbing element but expressed a confusion about how to proceed. I asked to see the rest of the crystallographic data -- specifically the atomic positions. I was shown a printout of some stuff from webelements.com or some similar web compilation of information about the elements. Sure enough, the lattice constants were given for the oxide form, but neither complete crystallographic data nor a crystallographic reference were listed. As everyone here should know, feff requires a list of atomic coordinates as its input data. To get a feff calculation, you must tell feff where the atoms are. Ifeffit and Artemis rely upon feff to provide theoretical fitting standards, thus to start fitting you need somehow to run feff. It is a syllogism, then, that you need some kind of list of atomic coordinates before you can begin fitting with Ifeffit and Artemis. With but a passing understanding of crystallography, it should be clear that you cannot generate a list of atomic coordinates given *only* the space group and lattice constants of a crystal. The space group and lattice constants tell you the shape and size of the unit cell, but, by themselves, do not tell you what is inside the unit cell. You see, a unit cell is just like a birthday present. The box might be big and pretty, but it's not a very good present until you open it up and find out what's inside. So........ if you intend to start your analysis by using Atoms, you have to find the COMPLETE crystallographic data. Often, this is one of the more difficult parts of the analysis problem. Sometimes crystallography data is hard to find. Matt keeps a big pile of atoms.inp files on one of his web sites and that helps a lot. Many libraries have copies of Wyckoff -- that also helps a lot. Google can be awfully handy -- indeed I ended up finding the missing crystallographic data earlier today with the help of Google. Often times you will be reduced to an exhausting search through the crystallography journals looking for the data you need. Other times you will never find what you are looking for and will just have to use something similar (but not the same) as your starting point. I presume it was already abundantly clear to everyone reading this message that the space group and lattice constants by themselves are not enough to use Atoms. But I needed to vent and so availed myself of this public forum. B P.S. The question of what to do when your sample is different from its crystalline form or when your sample is not a crystal at all has been discussed several times recently. If, after reading this little rant, that's your concern, you should read and respond to one of those threads rather than to this one. P.P.S. The next time you dig through the literature to find some obscure crystal structure, save someone else the effort. Be a hero, donate the atoms.inp file to Matt's database. -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 222 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/