NEB calculation

[Cyrille]

Dear all

I have some difficulties regarding a NEB calculation.
I am investigating the configuration of a rather complex molecule.
I have first performed DFT+U+VdW calculations and found two possible conformations of the molecule that I call C1 and C2.
The energy of C2 is 0.3 eV lower  than C1 and I want to calculate the energy barrier between these two conformations.
I have therefore generated a NEB path and run the calculation.

First of all I get several warning messages:
1)  WARNING: The Grimme DFT-D2 correction is only intended for GGA.PBE and GGA.BLYP exchange-correlation functionals.
2)  WARNING: Reactant configuration is not optimized.
3)  WARNING: Product configuration is not optimized

For the first warning I know it is related to VdW and DFT+U

But what is the meaning of warning 2 and 3.

In addition at the end of the calculation I find that the energy of C2 is now higher  than the one of C1..
I do not understand how it is possible since these two configurations have been fully relaxed in my preliminary DFT calculation.

Thanks a lot in advance

Cyrille

[Ulrik G. Vej-Hansen]

The meaning of the warnings in 2) and 3) is that the NEB optimization detects that your initial and final states are not relaxed to the level you have specified in the NEB script. We have observed that this warning can sometimes be triggered by numerical noise, so if the initial and final states are relaxed to just below the threshold, then the NEB algorithm might detect a slightly higher force, which is then just above the threshold. Say, the configuration is relaxed to 0.009 eV/Å with a threshold of 0.010 eV/Å and the NEB algorithm then detects a force of 0.011 eV/Å.

However, the reversal in the ordering of the energies suggest that is probably something else, but I cannot really say more without the script/other files.

[Cyrille]

OK I will try to run the scf calculation of end points with a more stringent convergence threshold on forces.
But if I understand well the NEB code is also running a preliminary convergence of end points to check if they are OK?

Cyrille

[Ulrik G. Vej-Hansen]

No, it just computes the forces from the existing density etc. in the nc. file, unless you specify preoptimization=True. See also the manual page: http://docs.quantumwise.com/manuals/Types/OptimizeNudgedElasticBand/OptimizeNudgedElasticBand.html

[Cyrille]

I am still struggling with the NEB calculation. I have converged the initial and final states with a more stringent criterion on the force (below 0.01eV/A) . However I still get the following message:

 WARNING: Product configuration is not optimized.                             |
|          Max. atom force is     3.528e-02 eV/Ang.                            |
| This means the product energy is most likely not a local minimum. 

Any idea what is going wrong?

I attach my python script.

thanks in advance

Cyrille

[Ulrik G. Vej-Hansen]

Is there any chance that there can be a slight difference in the calculator or optimization parameters between this script and the one you have used for optimizing the end-points? That is really my best guess based on what I see so far. The difference between the threshold you have specified and the printed force is not that big, but still bigger than I would expect if it was just numerics.

I will be happy to test if you can provide the .py, .log and .nc files from the optimizations of the initial and final configurations.