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

[David Foster]

Dear Serge
Thank you for your help. I think the best judge for this issue is experimental data for adsorption process, as you noted.

Thank you Nicola, Cyrille, and Serge for your helps.

Regards

David Foster

Ph.D. Student of Chemistry

--------------------------------------------
On Fri, 12/5/14, Serge Nakhmanson <smn at ims.uconn.edu> wrote:

 Subject: Re: [Pw_forum] Slab optimization with -24kbar  stress!!!
 To: "PWSCF Forum" <pw_forum at pwscf.org>
 Date: Friday, December 5, 2014, 7:36 PM
 
 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/
 
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