[Pw_forum] NEB vs 'direct' approach

[Guido Roma]

Dear Pawel,

I am not familiar with CdTe, but I wouldn't be surprised if the energy 
profile that you are looking at shows a double peak structure. In that 
case, as suggested by Stefano, you can miss the real saddle point and 
only sample an intermediate local minimum, by such a coarse sampling (I 
assume that you take 1 point also for the calculation using constraints, 
that you call direct). In some comparison that I did for a different 
system (cubic SiC) I got the same saddle points with NEB or constrained 
relaxations, provided that I had enough points in both (I was using 
simple NEB, not CI).

Guido 

Pawel Jakubas wrote:
> Dear Pwscf Users,
>
> I have started using CI NEB method implemented in espresso. I deal with
> native defects in bulk materials; let's say Cd interstitial in CdTe, which 
> crystalizes in zinc blende structure.
> It is known that it diffuses along [111] direction between tetrahedral 
> sites.
>
> Previously, to calculate barriers for diffusion I used 'direct' approach (I
> coined this name), which means I calculated total energies by fixing the
> position of an interstitial at a number of sites between metastable sites,
> and allowing all other atoms to relax (except one located at the outskirts
> of supercell, i. e., far away form the defect). The barriers heights
> calculated using NEB are usually higher than those obtained by 'direct'
> approach. I try to understand these differences. Because the diffusion 
> occurs along [111] direction in straight line, so the path is to some extent 
> in this case trivial (I am stressing it, because it is ok to choose nstep=1 
> in neb).
>
> Is there a clear, unanimous explanation why it is so? Are you familiarized
> with any paper in literature giving discussion of the dicrepancies between
> NEB results and those obtained using 'direct' approach? What is the physics 
> behind it?
>
> Thank you very much in advance for any comments, hints and help.
>
> Pawel Jakubas
> PhD Student
> Piotr Boguslawski Group
> Institute of Physics
> Polish Academy of Sciences
> AL. Lotnikow 32/46
> PL-02-668 Warsaw, POLAND
> group webpage: http://www.ifpan.edu.pl/SL-3/
> my webpage: http://www.ifpan.edu.pl/SL-3/PJakubas/jakubas.html
>
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