Hi Enyuan,
I don't really know a rule of thumb for ionic bond v.s. covalent bond, but
by default, the FEFF code will automatically choose reasonable muffin tins
in most cases, and most of the time it overlaps them by ~15%, i.e. FOLP
1.15. In many cases, the overlap doesn't change the structure in the XANES
a lot until you get far from FOLP 1.0. However, there are some special
cases. For example, we have found that using FOLP 0.8 for hydrogen atoms
seem's to work reasonably well in the XANES. This is because of the short
bond lengths of H. In addition, there are cases where one atom is fairly
isolated from the others, i.e., cage like structures, where FEFF can't
overlap the potentials, and setting FOLP to something smaller than 1.0 is
essential to make the code run. I would start by letting the code choose
the muffin tins automatically, then change them and see how the results
change. If the changes in the XANES are small, you probably don't have to
worry about FOLP. Otherwise, this is probably a limitation of the spherical
muffin tin potential. The best thing to do in that case, is to compare the
DOS (LDOS) with a calculation from a full potential electronic structure
code, i.e., WIEN2K, ABINIT, QUANTUM ESPRESSO, etc.
This can give you confidence in the FEFF calculation, or lead you to the
best values for FOLP. You could also use the FOLP values to get the best
comparison to experiment, but in some cases you may be fitting many-body
effects, which can lead you to the wrong conclusions about your system.
Cheers,
Josh Kas
On Mon, Nov 26, 2012 at 7:53 AM,
Re: