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

[Laurent Pizzagalli]

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