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

BARRETEAU Cyrille cyrille.barreteau at cea.fr
Fri Dec 5 11:27:53 CET 2014

I mean that when you do your "relax" calculation of the slab. If the threshold on the force is small enough the stress in the z direction (perpendicular to the surface) should be close to zero.
However since you have fixed several layers of your slab and it will bring some "extra" stress...
But in the plane (x,y) of the surface the stress will never be zero. This corresponds to the surface stress..


Cyrille Barreteau
CEA Saclay, IRAMIS, SPEC Bat. 771
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De : pw_forum-bounces at pwscf.org [pw_forum-bounces at pwscf.org] de la part de David Foster [davidfoster751 at yahoo.com]
Envoyé : vendredi 5 décembre 2014 11:11
À : PWSCF Forum
Objet : Re: [Pw_forum] Slab optimization with -24kbar stress!!!

Dear Nicola and Cyrille

Thanks for your help. But:

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.
    As Nicola said, it is possible to relax in-plane lattice constants, but what do you mean with this sentence:
                              "you should zero stress in the direction perpendicular to the surface"
    Do you mean relaxation in z-direction?!! It is vacuum, and if I do it, the vacuum thickness reduces and this is not my purpose.

    I converged smearing, k-point, wave function energy-cutoff and ... in the unit cell with 1mRY of total energy, and then used appropriate k-point
    for 2*2 surface slab, i.e., for z-direction I used 1 (due to the vacuum in z), and for x and y directions, I used half of the k-points for conventional cell
    (because of double-sizing "a" and "b" in 2*2 surface slab). Do you mean that I should converge them with stress value, too?

2- Nicola, Yes, It seems that I need to use z-constrained vc-relax to minimize the in-plane stress. I will do it, and check its stress.
   But for your second comment, I constructed the slab by using fully optimized conventional cell (with vc-relax command). I don't think their plane
   are very close. About your final comment, Do you mean I should change the atoms positions in the z-direction and repeat z-constrained vc-relax?


   David Foster

   Ph.D. Student of Chemistry

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