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

Stefano de Gironcoli degironc at sissa.it
Fri Sep 28 18:15:59 CEST 2007

Dear all,

I just want to add my two cents to this very nice discussion.

Possibly the answer is different depending on whether the pressurizing 
medium is in the gas or in the liquid/solid phase.

In the former case I would tend to agree with Matteo Cococcioni, at 
least if the average time between impacts is longer than typical 
electronic relaxation times (femptosecond), while I would tend to agree 
with Nicola Marzari for liquid/solid pressurizing media since in that 
case I would expect the contact to be rather continuous and constant, or 
at least fluctuating around some average value ...

Stefano de Gironcoli - SISSA and DEMOCRITOS

Hongjun Xiang wrote:
> 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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