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

Fri May 28 06:24:19 CEST 2021

Dear Laurent,

In Environ you can introduce a pressure term defined on a variety of interfaces. In particular, one can specify an electronic interface, similar to the one of Cococcioni et al., but based off a reformulation (from our J Chem Phys paper of 2012). But you can also specify an ionic interface, defined following the soft-sphere approach of Fisicaro et al, JTCT doi: 10.1021/acs.jctc.7b00375. The first case would introduce a ‘pressure' on the electrons via a contribution to the Kohn-Sham potential, the second case would introduce explicit forces on the ions.

There is a third interface, dubbed system, which corresponds to analytic functions (a plane, a cylinder, a sphere). Right now this latter kind of interface is rigid, has no direct effect on forces or Kohn sham potential, but it could be modified easily.

Best,

Oliviero

On May 27, 2021, at 3:59 PM, laurent pizzagalli <laurent.pizzagalli at univ-poitiers.fr<mailto:laurent.pizzagalli at univ-poitiers.fr>> wrote:

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
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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

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