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

David Foster davidfoster751 at yahoo.com
Fri Dec 5 16:36:17 CET 2014

Dear Cyrille, Nicola, and Serge
Thanks for your helps.
My main problem comes back to the computational method section in scholarly papers.
In adsorption of molecules on surfaces, authors say that they cleave a surface from fully optimized (vc-relax) unit cell, enlarge it 2*2 or
3*3 times in u and v directions, and then define 15-20 angstrom vacuum in the z direction. 
Then all of them take 2-4 bottom layers fixed to represent bulk, and only relax (and not vc-relax) other atoms containing 
adsorbed molecule. 
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?
Serge, maybe in the links you have provided this issue has been addressed. I will read them.


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, 5:24 PM
     On 12/5/14 5:11 AM,
 David Foster wrote:
 1- Cyrille, I fixed both in-plane and z-direction lattice
 constants. I need to fix the 15A vacuum in the z to prevent
 periodic interactions for studying adsorption.
     Dear David,
     Just my 2 cents to add to this.
     (a) Residual surface stress is a perfectly
 "legal" quantity. Some
     people hunt for it (as well as for the surface elastic
     specifically with DFT. You can check it out here:
     or here
     It is a lot of fun! :)
     (b) You have to be *extremely careful* when attaching
 something (a
     molecule?) to your surface, as this will open a
     new can of worms. You will need to check if your
 combined system is
     likely to have a dipole moment pointing perpendicularly
     the slab -- and it might. Then you may have to keep an
 xy mirror
     plane running through the middle of your slab [adding
 many more
     atoms to the calculation] or apply a dipole correction
     calculation convergence much slower]
     to get rid of it. If you don't, you run a chance of
 having a
     spurious electric field in vacuum due to interaction
 between the
     polarized slab and is periodic images.
     There is an old paper that comments on the situation:
     but much more had been published since then.
     The really graceful way of studying slabs involves using
 a code that
     can do 2D PBC in plane and break periodicity along
     perpendicular direction. But then you have to abandon
 the PW basis
     -- i.e., codes like VASP and QE -- and use something
         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 
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