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 > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > -------------- next part -------------- A non-text attachment was scrubbed... Name: guido_roma.vcf Type: text/x-vcard Size: 260 bytes Desc: not available Url : http://www.democritos.it/pipermail/pw_forum/attachments/20080206/5364ec34/attachment.vcf
