Scott, I use a P1 cell with parameters a=10.0001 b=10.0002 c=10.0003 alpha=90.001 beta=90.002 gamma=90.003 with no complaints. There is no cubic group with no symmetry (actually, every space group has some symmetry elements, for P1 it's only one - identity). It is theoretically possible to have a unit cell with 'cubic-like' parameters and triclinic symmetry, but quite unlikely. However, some people like that idea. There was a paper which proposed a triclinic symmetry and pseudocubic cell for pyrite to explain the anomalous anisotropy of pyrite in reflected light. So there you have it. Juro On Sun, 2 Oct 2005 11:31:30 -0400 (EDT) scalvin@slc.edu wrote:
Hi Bruce and others,
Back in the day, I used to use a "trick" for generating feff files for non-crystalline, molecular substances (or small nanoclusters). I'd use a space group of P 1 with a = b = c = 100, and alpha = beta = gamma = 90...then I could enter coordinates in hundredths of an Angstrom.
But I tried the trick recently, and the Atoms inside Artemis complains that I should be using a cubic space group and doesn't run.
I don't know my space groups very well, but is there actually a cubic group that doesn't have any symmetry within the unit cell? It seems to me that a real material (not just my trick) could have an "accidentally" cubic unit cell without being a cubic space group. Can someone who knows crystallography better than I do confirm that?
If so, then atoms could issue a warning, but should not prevent a feff.inp file from being generated.
Incidentally, of course I know there's a fairly trivial alternative to my trick...I could just create the feff.inp file without using atoms (Artemis even conveniently provides a template). But I think it's marginally easier for some of those new to the software to use the P 1 method, because then they don't have to think about potential lists and such.
--Scott Calvin Sarah Lawrence College
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---------------------------------- Juraj Majzlan Institute of Mineralogy and Petrology Albert-Ludwig-University of Freiburg Albertstrasse 23b Freiburg, D-79104, Germany telephone +49-761-203-6416 fax +49-761-203-6407 http://www.minpet.uni-freiburg.de/sites/personen/majzlan.html