[Pw_forum] BUG in NEB calculations with tefield=true

[Maxim Skripnik]

Dear developers,

I'd like to report a strange behavior of neb.x when one sets
tefield=true and dipfield=true. The error of the first image becomes
very large even though the xyz coordinates in FIRST_IMAGE are properly
relaxed. This seems to be not an issue for the actual neb calculation
since neb.p finds the energy barrier anyway. Nevertheless, it leads to a
corrupt .path file, which contains either ********** or/and NaN. When
one wants to restart the neb calculation (e.g. when going from
CI_scheme='no-CI' to 'auto'), is crashes since neb.x reads the corrupted
.path file which contains ******* and NaN.
I took a look at the pw.x calculation of the first image, which is done
only once at the beginning of a neb calculation (stored in
prefix/neb_1/PW.out). And already there one can see the problem:

end of prefix/neb_1/PW.out:

...
...
...

      convergence has been achieved in  26 iterations

      Writing output data file neb.save

Forces acting on atoms (Ry/au):

      atom    1 type  2   force = ************** NaN**************
      atom    2 type  2   force = ************** NaN**************
      atom    3 type  2   force = ************** NaN**************
      atom    4 type  2   force = ************** NaN**************
...
...
...

      Total force =          NaN     Total SCF correction = 0.000109

Interestingly, this doesn't happen with the last image, which was
relaxed with exactly the same pw.x settings. This happens even with
eamp=0, as long as tefield=true and dipfield=true. I was able to
reproduce the behavior with QE 6.0 and 6.1 on two totally different
computer clusters. Any idea why this happens?

Best regards
Maxim Skripnik
University of Konstanz