[Pw_forum] NEB first_last_opt

[Giuseppe Mattioli]

Dear Aleksandra

> I tried both ways and came to the conclusion that pre-optimization  
> pushes the system away from the minimum energy path (leading to the  
> increased barriers)

This is indeed the expected behavior. If your first and last images  
were potential energy minimum configurations, even if local ones, the  
pre-optimization 'relax' calculations would let them unchanged, or  
almost unchanged because only in the case of high symmetry systems  
input coordinates are "exactly" in a local minimum. This implies that  
your first and last images are not potential energy minimum  
configurations, and the code correctly optimizes them, and this of  
course lowers the potential energy and raises the potential energy  
barrier connecting the two configurations.

> In my opinion it is more physical if a migration corresponds to a  
> minimum energy pathway AND minimum restructuring.

I'm not sure I understand this sentence. If you don't start from/end  
to a local minimum why do you gain more physical insight into your NEB  
simulation? If you want to drive your system through some intermediate  
local minimum which permits the lowering of the global barriers across  
the PES, you can use the intermediate_image feature. If in the end you  
find the same high barriers then three interpretations can be given:  
1) your initial physical guess is wrong and in the "real" system  
vacancies do not exist/do not migrate; 2) The DFT level of theory used  
to perform the NEB calculation is not accurate and does not provide  
reliable numbers for transition states/barriers; 3) the migration  
occur through some redox process in which there is a change in the  
charge state of the defect which lowers the barrier. You are the only  
one that can think through this...
HTH
Giuseppe

Quoting Aleksandra Oranskaia <aleksandra.oranskaia at kaust.edu.sa>:

> Hello dear QE users,
>
> I have a question regarding pre-optimization of starting and ending  
> points of NEB.
>
> Dealing with multi-scale simulation of a vacancy migration in a huge  
> supercell (say, 350-400 atoms), should one make a pre-optimization  
> of starting and ending points of a path?
>
> I tried both ways and came to the conclusion that pre-optimization  
> pushes the system away from the minimum energy path (leading to the  
> increased barriers), because pre-optimization implies too deep  
> restructuring towards deeper minimums rather than the local minimums  
> closest to the minimum energy pathway.
> In my opinion it is more physical if a migration corresponds to a  
> minimum energy pathway AND minimum restructuring.
>
> If you would simulate vacancy migration in a huge supercell with NEB  
> optimization, would you optimize starting and ending points? Why?
>
>
> Thanks in advance and hope to hear your opinions and reasonings,
>
> Regards,
> Alex.
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    Giuseppe Mattioli
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