[QE-users] NEB calculation issue with 'fixed' atoms

[Giuseppe Mattioli]

Dear Laurent
AFAIK, in NEB you can only fix atoms once, in the initial image, and  
keep them fixed across the whole reaction coordinate (no need to add 0  
0 0 in images 2...final).
HTH
Giuseppe

Pietro Delugas <pdelugas at sissa.it> ha scritto:

> Dear Laurent
>
> 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.
>
> I am just guessing though. I should look in the code to see if this  
> is   what's actually  happening.
>
> In the meantime you could try  set
>
> 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.
>
> hope it helps
>
> regards - Pietro
>
> On 21/10/19 10:08, Laurent Pizzagalli wrote:
>> Dear all,
>>
>> 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.
>> 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.
>>
>> 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.
>>
>> Any help or suggestions are welcome,
>>
>> Best regards
>>
>> L. Pizzagalli
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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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