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

[Matteo Cococcioni]

Dear Xiang,

let me add something to what Nicola wrote.

when a system is under pressure also its electronic structure is under 
pressure. In fact pressure is transferred to the system
by electrostatic and Pauli repulsion interactions with the electrons of 
other atoms/molecule in the pressurizing environment of your system. 
However in the electronic enthalpy method to impose the pressure, the 
effect of the collisions with external atoms or molecules is mimicked by 
a smooth potential of the charge density (like if the colliding 
particles were smeared in a continuum)
that is meant to produce the same average effect. This turns out to work 
quite well for the structure of the system. The surface electronic state 
however could be distorted (compressed).
To answer to your question (not easy) I suppose one should think of what 
happens to the system between collisions with external atoms/molecules 
of the environment. do the electrons relax to their instantaneous ground 
state corresponding to a compressed ionic configuration? I would guess 
yes, and probably they do before ions have time to do the same. so I 
have a preference for answer (2). Anyway this is just my guess, I have 
not much experience with the electronic relaxation once you release 
pressure in the compressed configuration.

Hope this helps,

Matteo



Nicola Marzari wrote:
> 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
>
> ---------------------------------------------------------------------
> Prof Nicola Marzari Department of Materials Science and Engineering
> 13-5066 MIT 77 Massachusetts Avenue Cambridge MA 02139-4307 USA
> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
> ------------------------------------------------------------------------
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