[Pw_forum] vc-relax & cell_dofree

[Dr. Thomas Brumme]

Well, it should work and do exactly what it says but it won't respect  
the hexagonal symmetry which might be problematic if you want to  
enforce it. What do you mean with "not appropriate"?
You could also relax the in-plane constant by hand and in this way  
keep the hexagonal symmetry - just make several scf calculations with  
different in-plane constant and fit the total energy with a quadratic  
function to find the minimum...

Another point I noticed - if you want to calculate a 2D MOF maybe the  
metal center doesn't want to stay in the highest symmetry position;  
something like a Jahn-Teller distortion. Maybe the relaxation of the  
cell falls into a wrong minimum because of this. Can you try a  
relaxation without symmetry? Or, maybe just comparing the forces and  
stresses after an scf with symmetry and without to see if this changes  
something...

Thomas


Quoting BARRETEAU Cyrille <cyrille.barreteau at cea.fr>:

> Thanks for your reply but I am still not very sure I understand well  
> the difference between the two options:
> 'xy' :     only v1_x and v2_y are moved
> '2Dxy' :     only x and y components are allowed to change
>
> When the vectors v1 and v2 are not orthogonal (which is my case) the  
> option 'xy' is not appropriate ?
>
> Cyrille
>
> ========================
> Cyrille Barreteau
> CEA Saclay, IRAMIS, SPEC Bat. 771
> 91191 Gif sur Yvette Cedex, FRANCE
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +33 1 69 08 38 56 /+33  6 47 53 66 52  (mobile)
> email:     cyrille.barreteau at cea.fr
> Web:     http://iramis.cea.fr/Pisp/cyrille.barreteau/
> ========================
> ________________________________
> De : pw_forum-bounces at pwscf.org [pw_forum-bounces at pwscf.org] de la  
> part de Thomas Brumme [thomas.brumme at uni-leipzig.de]
> Envoyé : mardi 25 juillet 2017 11:32
> À : pw_forum at pwscf.org
> Objet : Re: [Pw_forum] vc-relax & cell_dofree
>
>
> Dear Cyrille,
>
>
> from the input description:
>
> 'xy' :     only v1_x and v2_y are moved
>
> '2Dxy' :     only x and y components are allowed to change
>
> Thus, even in the case of cell_dofree='xy' I think the cell will  
> change. Anyway, the deviation from 120 DEG is quite small (119.43  
> DEG if I'm not wrong) - I don't know if this could be do to  
> numerical issues. Maybe try starting a new relaxation with a  
> hexagonal cell but smaller in-plane lattice vector of ~99.1%
>
>
> Cheerio
>
>
> Thomas
>
> On 07/25/17 09:08, BARRETEAU Cyrille wrote:
> Dear QE community
>
> I have a question related to vc-relax and the option cell_dofree.
> I want to relax a 2D metal-organic framework (MOF) with hexagonal  
> lattice (ibrav=4)
> Could you please explain what is the difference between  
> cell-dofree=xy and cell_dofree=2Dxy?
>
> I have noticed that when using cell_dofree=2Dxy the final  
> CELL_PARAMETER is no longer perfectly hexagonal (see below). Should  
> I use cell_dofree=xy instead and why?
>
> final:
> CELL_PARAMETERS (alat= 25.44640422)
>    0.990921488  -0.000187347   0.000000000
>   -0.495622997   0.858299558   0.000000000
>    0.000000000   0.000000000   0.742630080
>
> Thanks in advance
>
> Cyrille
>
> ========================
> Cyrille Barreteau
> CEA Saclay, IRAMIS, SPEC Bat. 771
> 91191 Gif sur Yvette Cedex, FRANCE
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +33 1 69 08 38 56 /+33  6 47 53 66 52  (mobile)
> email:     cyrille.barreteau at cea.fr<mailto:cyrille.barreteau at cea.fr>
> Web:     http://iramis.cea.fr/Pisp/cyrille.barreteau/
> ========================
> ________________________________
>
>
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> --
> Dr. rer. nat. Thomas Brumme
> Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry
> Leipzig University
> Phillipp-Rosenthal-Strasse 31
> 04103 Leipzig
>
> Tel:  +49 (0)341 97 36456
>
> email: thomas.brumme at uni-leipzig.de<mailto:thomas.brumme at uni-leipzig.de>