[QE-users] NEB calculation issue with 'fixed' atoms

[Pietro Delugas]

Dear Laurent

It might be that the  fixed-position acts only at the "engine"  level 
inside pw, the forces are then are sent to the path level and there the 
longitudinal component  is changed accordingly to the neb algorithm you 
are using,  if the neb part of the program is unaware of the fixed 
positions then these atoms could be moved.  When fixed positions are 
equal for all the images this is not an issue as distances on that 
coordinate are all zero and the longitudinal reaction will also be 0.

I am just guessing though. I should look in the code to see if this is   
what's actually  happening.

In the meantime you could try  set

use_masses =  .true.  in the &path namelist and then specify fictitious 
masses,  something like 1 for atoms which are free to move and 10000 for 
the fixed ones.

hope it helps

regards - Pietro

On 21/10/19 10:08, Laurent Pizzagalli wrote:
> Dear all,
>
> I encountered a weird and unexpected behavior when running a NEB 
> calculation with QE. In this calculation, several (boundary) atoms are 
> fixed. All initial images were provided in the input file, with the 
> required flags "1 1 1" for mobile and "0 0 0" for fixed atoms. 
> However, during NEB iterations, the positions of the fixed atoms are 
> updated with new values, which is obviously something I'd like to 
> avoid. It appears in all images, except in the first one.
> I do not understand the issue, because in the 'PW.out' output files 
> contained in the subdirectories associated to each image, one can 
> clearly see that the '0 0 0' flags are correctly set for the fixed 
> atoms, for all images, and are conserved throughout the calculations. 
> Still, the coordinates of these atoms are evolving. Moreover, in the 
> .crd ouput file in the main directory, which includes the updated 
> coordinates for all images during the NEB calculations, only the fixed 
> atoms of the first image have the '0 0 0' flags.
>
> One specificity of my calculation is that the (x,y,z) positions of the 
> fixed atoms change from one image to the other (but the ordering of 
> the atoms in each image is the same). This is not common, and I was 
> wondering whether this could be the cause of my issue? If not, I do 
> not have a clue why I got this weird behavior.
>
> Any help or suggestions are welcome,
>
> Best regards
>
> L. Pizzagalli
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