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I have some general questions about doing the structure relaxation of a thin film structure in VASP. I want to study the how the band structure evolves as we increase the number of layers of the material. The spin-orbit coupling (SOC) was included in the calculation, and I encounter the following question.

  1. Should I turn the ISYM = 0 to also change the shape of the unit cell during the structure relaxation?

  2. Should I include the SOC during the relaxation?

  3. I saw some people doing this structure relaxation will fix the bottom layer of the slab by using the selective dynamic option, when should I use this technique?

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Should I turn the ISYM = 0 to also change the shape of the unit cell during the structure relaxation?

In essence, you are asking how to build a slab model. One special point is that you should add a large vacuum layer (here I assume that direction is along [001]). The relaxation of the slab model is limited to (100) and (010), so you can't change the shape. Namely, if want to use ISIF=3, you should modify the source file [constr_cell_relax.F] and re-compile your VASP.

Should I include the SOC during the relaxation?

In general, the SOC can be ignored in relaxation.

I saw some people doing this structure relaxation will fix the bottom layer of the slab by using the selective dynamic option, when should I use this technique?

When you study the surface science.

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  • $\begingroup$ Thank you for your answer, Jack. May I ask further question about this ?So like what you said, the ISYM should be 2 during the relaxation procedure ? And the z-position of the atom ((thickness of the slab) should be fixed during the relaxation ? $\endgroup$
    – JensenPang
    Commented Jan 25, 2021 at 13:26
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    $\begingroup$ Although I would generally agree that SOC can usually be ignored in relaxations, there are actually a few examples of materials in which you get the incorrect structure/symmetry if you don't include SOC -- so it is always worth cross-checking. $\endgroup$
    – ProfM
    Commented Jan 25, 2021 at 13:33
  • $\begingroup$ @JensenPang If the lattice constant is identified, of course, you can relax with ISIF=2. For slab calculation, you can first fix the lattice constant by relaxing a bulk structure. $\endgroup$
    – Jack
    Commented Jan 25, 2021 at 13:48
  • $\begingroup$ @ProfM Could you detail this with an answer? $\endgroup$
    – Jack
    Commented Jan 25, 2021 at 13:49
  • $\begingroup$ @Jack I am not sure this is worth a full answer, but here are two papers (disclaimer: I am a co-author in one): iopscience.iop.org/article/10.1088/2515-7639/ab4c2b journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.117701 $\endgroup$
    – ProfM
    Commented Jan 25, 2021 at 14:09

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