[Wien] negative value of U in solids

[Peter Blaha]

I don't believe that the physics with negative U in semiconductors is 
correct, even if it gives "good gaps".

The GGA+U (SIC) methods shifts states up/down in energy with respect to 
their occupation. In that it expects implicitly that a "fully occupied 
state" has charge = 1 and it will shift states with occupation less 
(more) than "0.5" up (down) in energy.

However, for these delocalized electronic states of Si or Ge(s,p) (not 
Ge-d) even a fully occupied orbital may have an occupation inside sphere 
of less than 0.5 and would get an up-shift. To "correct" for this 
spatial decomposition problem, one could set a negative-U, but ...

On 07/08/2015 07:38 PM, Michael Sluydts wrote:
> Hello Sandeep,
>
> A negative value of U for instance occurs in Ge.
>
> I was given this article before to explain this behavior:
>
> http://www.sbfisica.org.br/bjp/files/v36_286.pdf
>
> Note that the effective U = U-J.
>
>
> Best regards,
>
> Michael Sluydts
>
> sandeep schreef op 8/07/2015 om 18:49:
>> Dear users!
>> It may not be appropriate to post this question here,
>> "Did any one encounter negative value of U in solids (compounds) ? if
>> so explain its origin and quote related concepts. Kindly explain the
>> origin and experimental supports if any?"
>>
>>
>>
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>
> --
> ir. Michael Sluydts
> Center for Molecular Modeling
> Ghent University
> Technologiepark 903
> 9052 Zwijnaarde, Belgium
> tel. +32 (0)9 264 66 19
> https://molmod.ugent.be
>
>
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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