[Pw_forum] Slab optimization with -24kbar stress!!!

[Serge Nakhmanson]

On 12/5/14 10:36 AM, David Foster wrote:
> If I do according to the published works, I get large stress, as it was discussed in previous long discussion.
> If I do according to Nicola's advice, the stress eventually reduces, but the bottom layers positions will not be the bulk ones.
> Which one is correct?
>
David,

I do not think you are asking the right question here.

IMO, the appropriate question is: what elastic condition on my system do I want 
to simulate?

If you want to do a surface that is rigidly clamped to the bulk, you will get 
(correctly!) non-zero residual stress.
BTW, a 24 kBar stress is not large, unless you got a very soft system like a 
polymer/molecular crystal.

If you want to do a surface that is completely relaxed, do a vc-relax on your 
slab and get rid of the stress completely.

Finally, you can also do a partially relaxed surface and get a stress that is 
somewhere in between.

I am not an expert on CeO, but could imagine that all of these cases may be 
appropriate -- depending on what you
are after. The latter is for you and your advisor (I hope you got one?) to 
figure out. Usually you have to take hint
from experiments, i.e., find out what sort of films and surfaces 
experimentalists are dealing with, and then
decide how you can mimic this in your simulation.

S.

-- 
Serge M. Nakhmanson
Dep-t of Materials Science & Engineering, and Institute of Materials Science
University of Connecticut
Storrs, CT 06269-3136
Phone: (860) 486-5252
http://satori.ims.uconn.edu/