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I have two solid surfaces that I want to combine to study the interface as seen in the attached image. What I know is the lower part of the bottom structure is fixed to mimic the bulk. However, do I have to do the same for the top layer too? Where by I have to fix the top part to mimic the bulk as I have shown in the diagram? Or the top layer need no bulk part? Your response would be deeply appreciated. Thank you.

enter image description here

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  • $\begingroup$ @ShaunHan can you help me with this? $\endgroup$ Aug 19, 2023 at 6:35
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    $\begingroup$ Hi, assuming a periodic calculation, e.g. pw-DFT , the problem one encounters in interfacing bulk crystals is that there is no defined interface, but even what is considered bulk, by the code itself, is nothing more than an additional interface, where material A is to some degree surrounded by material B. Although there are works in the literature that perform similar calculations, great care must be taken with the choice of individual bulk crystals and exposed faces (hkl). In any case, it is quite obvious, after a few attempts, why the slab-on-slab approach is typically used. Hope this helps. $\endgroup$
    – Salvios
    Aug 25, 2023 at 17:42
  • $\begingroup$ @Salvios I think my question hasn't still been answered. I wanted to find out if I have two model two bulks like above or one for the bottom layer only $\endgroup$ Aug 25, 2023 at 22:45
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    $\begingroup$ This relates to my previous answer, if you are modelling a surface (slab on slab + vacuum) you don't need the a bulk top layer because the whole upper layer is relaxed. In case a bulk interface is what you need, yes, you should have bulk sections for both. But as i explained things get complicated quickly, at least in my experience, because a lot of stress is present and many ions need to be omptimized. $\endgroup$
    – Salvios
    Aug 27, 2023 at 9:56
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    $\begingroup$ @Salvios I made it slab on slab + vacuum. Thanks very much. $\endgroup$ Aug 27, 2023 at 21:18

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For bulk interface you need to freeze both atoms at the bottom and those at the top. For a slab model all the top layers are relaxed. @Salvios answer was correct but it was added as a comment and not an answer.

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