[Pw_forum] Is Electronic-Enthalpy Functional for Finite Systems Under Pressure implemented in espresso-3.2?

[Nicola Marzari]

Hongjun Xiang wrote:
> Dear Dr. Cococcioni and others,
> I tried some test calculations and realized that the pressure changes 
> the electronic structure and thus
> decreases the volume of the system. If I am interested in the band gap 
> of a finite system under pressure, which method of the followings is 
> correct?
> (1) Relax the structure under the pressure, and calculate the gap using 
> the relaxed structure under the pressure.
> (2) Relax the structure under the pressure, and calculate the gap using 
> the relaxed structure without the pressure.
> 
> It turns out the results from the above procedures are different.
> 
> Thank you very much.
> 
> Best regards,
> Hongjun Xiang


Dear Xiang,

1) is the correct procedure - but be aware that the band gap is
not a property that you can address in DFT - not in principle, and
largely not in practice. At best, you could look at DFT predictions
of, say, the derivative of the band gap in bulk semiconductors
with respect to biaxial strain, and see how well you can do to predict
the slope. I think the error would be in the 20-30% range, a bit better
than the 100% error on the band gap.

				nicola

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Prof Nicola Marzari   Department of Materials Science and Engineering
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