[Ifeffit] P 1 space group

[Juraj Majzlan]

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 at 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