Re: Changing the oxidation state in FEFF

[Bruce Ravel <ravel@phys.washington.edu>]

On Thursday 01 April 2004 04:30 pm, Gilles Hug wrote:
> If you use a SCF cycle to calculate potentials it doesn't look very
> meaningful to change the oxidation state.
> The charge should be calculated during SCF.
> Gilles
>
> Le mercredi, 31 mars 2004, à 14:51 Europe/Paris, Wojciech Gawelda a
>
> écrit :
> > I was wondering whether it is possible to change the oxidation state
> > of the absorber in FEFF input file (is there any card that could be
> > helpful doing so)?


I wanted to expand a bit on what Gilles said.

The whole point of the self consistency is to determine charge
transfer, which (presumably) has a relation to oxidation state.  I
think that if you compute the SCF potentials for, say, Cr 3+ and 6+
you will find that there are different amounts of charge transfer.
How does Feff know to do that magic?  There is a relationship between
coordination and oxidation.  In the case of Cr, one is tetrahedral and
the otehr is octahedral.  You will also find that the XANES, which is
very different for the two Cr species, is computed rather well.

Of course, in neither case is a large integer number of electrons
transfered between atoms.  Feff typically finds that a fraction of an
electron is transfered (much like reciprocal space methods do, such as
LMTO).

In priciple, specifying the geometry should be enough to specify the
oxidation state.  It is certainly possible that your material might
not be well represented by spherical muffin tins or a flat interstice,
but I doubt that you will imporve the calculation by putting arbitrary
charge on certain atoms.

B


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