[Pw_forum] Slab optimization with -24kbar stress!!!
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
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.
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.
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:
It is a lot of fun! :)
(b) You have to be *extremely careful* when attaching
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
plane running through the middle of your slab [adding
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
spurious electric field in vacuum due to interaction
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
University of Connecticut
Storrs, CT 06269-3136
Phone: (860) 486-5252
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