[QE-developers] [EXT] Re: A question concerning a QE development

[laurent pizzagalli]

Dear Oliviero,

thanks for your email. I know the paper you mentioned, but I thought 
(wrongly apparently) that this approach was only implemented in cp.x 
(the &PRESS-AI namelist). Also I was not really sure this could be used 
for applying a uniaxial stress on an isolated system. The pressure is 
accounted for by considering the PV term in the enthalpy, with a volume 
computed from rho. In the case of a uniaxial compression, one should 
instead define a surface where the stress should be present? I did not 
see how it could be done. This is why I considered forces on ions 
instead of forces on electrons.
It would be interesting to study whether these different approaches 
could imply different mechanical behaviors !

Best regards

Laurent


On 27/05/2021 22:34, Andreussi, Oliviero wrote:
> Dear Laurent,
>
> I am not fully sure about the details of your implementation and I am 
> not commenting on the best place to put your keywords (as Paolo said, 
> Environ adopts an external file for its own input). I mostly wanted to 
> mention that Environ allows you to introduce an external pressure into 
> the system. With the right settings this could be used to apply a 
> compressive pressure on rigid planes above or below a slab or, using a 
> cylindrical symmetry, on a nanowire (but also to a isolated system, as 
> per Cococcioni et al, PRL 10.1103/PhysRevLett.94.145501).
>
> Best,
>
> Oliviero
>
>
> Oliviero Andreussi
> --
> Assistant Professor
> Department of Physics
> University of North Texas
> Email: oliviero.andreussi at unt.edu <mailto:oliviero.andreussi at unt.edu>
> Phone: +1-(940)-369-5316
> Skype: olivieroandreussi
> Web: https://www.materialab.org
>
>> On May 27, 2021, at 3:04 PM, Paolo Giannozzi 
>> <paolo.giannozzi at uniud.it <mailto:paolo.giannozzi at uniud.it>> wrote:
>>
>> Dear Laurent
>>
>> the proliferation of input variables in QE is an old and known 
>> problem. I am not sure what the best solution should be, though. One 
>> possibility I would consider is to read the needed variables from a 
>> separate file.
>> If I remember correctly this is what the "Environ" library does.
>> Another one is to extend the existing ATOMIC_FORCES card, that 
>> currently supports only a list of forces, to cover your case as well.
>>
>> Paolo
>>
>> On 27/05/2021 10:00, Laurent Pizzagalli wrote:
>>> Dear Dr. Giannozzi,
>>> I write to you concerning the implementation of an external force 
>>> field in quantum espresso. The motivation is the investigation of 
>>> the mechanical properties of nanoparticles, especially in the 
>>> plastic regime, obtained by uniaxial compression. The implementation 
>>> is not complicated, since you (the developers) had already planned 
>>> this potentiality in the file 'plugin_ext_forces.f90'.
>>> Actually, i already made the implementation in cp.x, but in a quick 
>>> way, in a local copy of the code. Since it is successful, I would 
>>> like to do it properly, merging it with the development version 
>>> using git. I got almost all the information I need in the developer 
>>> manual. But I still have one question, concerning the general 
>>> organization of the input. Right now, I implemented only one kind of 
>>> force field, fully repulsive. It allows for compressing the 
>>> nanoparticle between two planes. Six keywords are needed, one for 
>>> the activation of the force field, four to define the position of 
>>> the planes and the increment for their displacements at each 
>>> timestep, and a last one to define the strength of the repulsion. My 
>>> question is: what would be the best way to organize these keywords? 
>>> Originally, I put all of these into the &IONS namelist. Another 
>>> option would be in the &SYSTEM namelist. And finally, maybe it would 
>>> be better to create a specific NAMELIST to include these keywords 
>>> (something like &EXTFF)? I also plan to implement another force 
>>> field (including an attractive interaction), which would require 
>>> other keywords. So the creation of a specific NAMELIST seems the 
>>> best option in my opinion. But I would like to have yours before 
>>> proceeding.
>>> Best regards
>>> Laurent
>>
>> -- 
>> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
>> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>> Phone +39-0432-558216, fax +39-0432-558222
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<Laurent.Pizzagalli at univ-poitiers.fr>
http://laurent.pizzagalli.free.fr/            Tel +33 549 49 74 99
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