[Pw_forum] Calculation of U for Fe co-existing in two different oxidation state

[Nicola Marzari]

On 01/11/2013 07:40, Prasenjit Ghosh wrote:
> Dear all,
>
> I am trying to do some calculations for iron oxide, in which the Fe is
> present in both the oxidation states, +2 and +3. Can any one suggest how
> to determine the value of U for such cases?
>
> With regards,
>
> Prasenjit
>


Dear Prasenjit,

we had studied this for the ultrasoft PBE Fe in the QE database
(the one labeled "mit" - in the linear-response formulation U is really 
dependent from the pseudopotential - especially the oxidation state of
the all-electron atom - and the chemical environment where it is used).

We obtained 5.6 eV in both the ferrous and the ferric case
(P.H.-L. Sit et al. / Journal of Electroanalytical Chemistry 607 (2007) 
107–112), for iron in a 6-fold aqua cluster. These numbers may vary in
the solid state, and indeed it would be good to try and recalculate it 
along the lines that Matteo suggested int he previous post.

Note that the pseudopotential above had been generated in the 2.5+ 
oxidation state, but the results were only marginally better than a
standard one generated for a neutral atom (as it should, ideally, if
transferability holds). In all these cases, essential to have 16 valence
electrons (i.e. include the semicore 3s and 3p in the valence, even if
only the 3p are really needed).

				nicola



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-- 

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL