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<p><font size="-1">Dear Laurent <br>
</font></p>
<p><font size="-1">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. <br>
</font></p>
<p><font size="-1">I am just guessing though. I should look in the
code to see if this is what's actually happening. <br>
</font></p>
<p><font size="-1">In the meantime you could try set <br>
</font></p>
<p><font size="-1">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. <br>
</font></p>
<p><font size="-1">hope it helps <br>
</font></p>
<p><font size="-1">regards - Pietro </font><br>
</p>
<div class="moz-cite-prefix">On 21/10/19 10:08, Laurent Pizzagalli
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:fc4bf9f5-9421-28ea-87de-9f58c4d2c967@univ-poitiers.fr">Dear
all,
<br>
<br>
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.
<br>
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.
<br>
<br>
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.
<br>
<br>
Any help or suggestions are welcome,
<br>
<br>
Best regards
<br>
<br>
L. Pizzagalli
<br>
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