[Pw_forum] NEB vs 'direct' approach

[Pawel Jakubas]

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