[Wien] chemical bonding

[Laurence Marks]

Charge can be estimated using (as a conventional measure) the Bader
volume around an atom, for instance by using the aim command in
Wien2k.

However, this is not the same as "Fe2+" which has a strict meaning
that the iron atom is in a 2+ valence state. This is not the same (but
alas often confused with) the effective charge, which I prefer to
write as +X, as against valence as X+. You can blame Linus Pauling for
this as he used the word "charge" in his original papers, when
referring to what is really valence.

As an example, strontium titanate has a valence of Sr2+ Ti4+ (O2-)3.
However, the "charges" from a Bader analysis are closer to Sr+1.6
Ti+2.2 (O-1.2)3 . (The numbers are right, there is a slight rounding
error in the charges).

In the limit of completely ionic bonding the two numbers are the same.
However, in general they are not and a source of endless confusion.
Depending upon exactly what functional you use the charges change, for
instance PBE tends to be too covalent and hybrids and/or LDA+U
improves on this (for d-electrons in Wien2k) making oxides more ionic.

For an oxide, you can readily get the valence from a bond-valence
analysis. I am not sure about other cases.

And, just to make life more interesting, many spectroscopies (e.g.
XPS, EELS) have been calibrated versus valence.

2012/3/5 Yundi Quan <quan at ms.physics.ucdavis.edu>:
> Dear Professor Laurence Marks,
> Charge ordering has long been a generator of controversies. I'm wondering
> whether charge as measure from DFT can be used to identify charge ordering.
>
> Thanks
> Yundi
>
>
> 2012/3/5 Laurence Marks <L-marks at northwestern.edu>
>>
>> Charge, as measured from DFT, and valence (chemical bonding) are very,
>> very different.
>>
>> Without more information as to what you are asking, that is about the only
>> answer available.
>>
>> ---------------------------
>> Professor Laurence Marks
>> Department of Materials Science and Engineering
>> Northwestern University
>> www.numis.northwestern.edu 1-847-491-3996
>> "Research is to see what everybody else has seen, and to think what nobody
>> else has thought"
>> Albert Szent-Gyorgi
>>
>> On Mar 5, 2012 7:11 PM, "Yundi Quan" <quanyundi at gmail.com> wrote:
>>>
>>> Does LAPW method always underestimate the chemical bonding? For example,
>>> Fe2+ and Fe3+ have the same charge around them.
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-- 
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu 1-847-491-3996
"Research is to see what everybody else has seen, and to think what
nobody else has thought"
Albert Szent-Gyorgi