[Ifeffit] charge transfer by SCF in FEFF8 and simulation for charge disproportionation

[Scott Calvin]

Hi Ying,

I think maybe the problem isn't so much with FEFF as with a century of  
conventions in chemistry.

Until fairly recently, no one really knew where electrons were sitting  
in real materials. Chemistry "works" pretty well if you do things like  
assign formal oxidation states and apply octet rules. But as a  
physicist, you know that the actual wave functions aren't going to be  
that simplistic.

As both theory and experiment advanced, it became very clear that even  
the most stereotypically ionic substances don't actually consist of  
isolated ions interacting with their classically assigned charges. I  
guess you can say that implies a partially covalent character, but  
electrons are indistinguishable, so there aren't really individual  
"pairs" of electrons in covalent bonds either. The whole solid is  
actually a great big messy quantum mechanical entity.

That does not mean that models like formal oxidation state, chemical  
bonds, and the like aren't extremely useful. Personally, I consider  
myself a chemical physicist, not a solid state physicist, and I much  
prefer the bonding description to, for example, a model based on  
"bands" for understanding in my own head what is happening.

But the bottom line, to me, is that you aren't going to find those 1.7  
electrons that you think should be involved in the bonding somehow in  
the FEFF SCF calculation, nor do they really belong. FEFF, as I  
understand it, doesn't have interactions between electrons in  
different atoms at all (bonding and antibonding orbitals, if you like  
that kind of description), and thus only needs to know the net charge  
transfer. "Covalent bonds" have no meaning in FEFF.

OK, FEFF experts, how'd I do? :)

--Scott Calvin
Sarah Lawrence College


On Aug 28, 2008, at 4:03 PM, Ying Zou wrote:

> Dear all,
>
> I am doing a SCF calculation by Feff8.40 for a model compound
> PbTiO3. However, the obtained charge transfer deviates far away from
> its well-known formal oxidation state. According to common electron
> negativity data, the oxidation state would be:
>
> Pb  2+
> Ti  4+
> O   2-
>
> The SCF run gave out charge transfer for those atoms as following:
>
> Pb 0.717
> Ti 0.181
> O  -0.300
>
> After I read through the existing threads on charge transfer in this  
> list, I know that transfered charge FEFF gives out is just a portion  
> of general formal oxidation state. Can we undderstand that this is  
> just ionic portion for entire bonding? Such as for oxygen, "-0.300"  
> is ionic bonding, while the rest of valency (-1.700) exists as  
> covalent bonding, which doesn't have apparent charge transfer by  
> definition.
>
> Another related question is then: is there any way in FEFF to take  
> into account this "covalence" part? (maybe deriving from overlapping  
> area of mufftin-potential?) I think this may be crucial for a self- 
> consistent analysis of charge disproportionation.
>
> Any comments on this would be greatly appreciated. Attached please  
> see"atom.inp" and "feff.inp". Thanks so much!
>