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

[David Foster]

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?


   Regards

   David Foster

   Ph.D. Student of Chemistry