[CP2K-user] Wrong Endpoint Energy in NEB Calculation

[Martin Brehm]

Dear Community,

we performed a CP2K NEB calculation of a solid state system. The starting 
and ending configuration of the band are equivalent after wrapping 
coordinates into the primary image of the cell (just two atoms have changed 
place, one of them moving out of the primary image). This can easily be 
confirmed by running single point calculations of the starting and ending 
calculations - they yield identical energies in CP2k.

When we now perform the NEB calculation, the ending point unexpectedly 
yields a different total energy (too high by 13.1 kJ/mol). The starting and 
ending configuration cannot move during the NEB optimization, as we 
specified "OPTIMIZE_END_POINTS .FALSE.". And the energy of the starting and 
ending configuration remains constant during the NEB optimization - the 
strange higher energy appears from the beginning on. So this problem is 
probably not related to the NEB calculation itself.

Attached ("image_energy.png") you can find a plot of the total energy of 
the NEB images at some point during the NEB optimization, where the first 
and the last value should be identical.

We already checked the following:

(*) We tried to simply swap the starting and ending geometry. Then, again, 
the energy of the ending geometry is higher by the same amount (13.1 
kJ/mol), and the energy of the starting geometry is correct this time. This 
confirms that both configurations are identical. Probably there is a bug 
which affects the energy calculation of the last image in a NEB calculation.

(*) We were able to reproduce this error on different computer systems, 
using different CP2k versions, from 2016 to recent versions.

(*) Even the contribution of the dispersion correction to the last NEB 
image is different than to the first one. This indicates that the issue is 
not related to the SCF calculation, but that somehow the geometry of the 
last image is not correct in the internal data structures.

The following files are attached to this post:

image_energy.png - Plot of the total energy of the NEB images at some point 
during the NEB optimization.
band.inp - Input file for the NEB run.
start.xyz - Starting configuration (first NEB image).
end.xyz - Ending configuration (last NEB image).
coord.xyz - Again the coordinates for the "&COORD" section.
li12si7-BAND01.out - Beginning of the Log file for the first NEB image.
li12si7-BAND16.out - Beginning of the Log file for the last NEB image.

If you want to try to reproduce this strange behavior, you do not need to 
wait until the NEB calculation is converged. Already one or two geometry 
steps will be sufficient to check if the first and last (fixed) images have 
identical energies (as they should) or not.

Any recommendations are welcome.

Best regards,
Martin
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