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

[Hongjun Xiang]

Dear Prof. Marzari and Dr. Cococcioni,

Thank you for your replies.
Both opinions seem reasonable. I will think about this more seriously.

Best regards,
Hongjun Xiang

=============================================================
H. J. Xiang
Postdoctoral Research Associate
National Renewable Energy Laboratory
http://www4.ncsu.edu/~hxiang/
============================================================= 

Matteo Cococcioni wrote:
>
> 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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