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This is something I don't understand comprehensively. Also aren't default external pressure values equal to zero for DFT codes?

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    $\begingroup$ It depends on your situation. You can tweak parameters so that there is no external pressure applied - there is a distinction between external pressure that you apply and the force on the cell - the force on the cell is reduced during geometry optimization to obtain strain-free lattice constants. If your question is with regard to why one might need to apply pressure, there are a variety of reasons - For example, you might want to study how your material behaviour changes on application of a strain, or to model interactions between a substrate and molecule etc. $\endgroup$
    – Xivi76
    Jul 28 '20 at 17:59
  • $\begingroup$ Lets's say I just want to find the optimized geometry. Should I still apply it? $\endgroup$
    – Alfred
    Jul 28 '20 at 18:03
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    $\begingroup$ If you just want to relax the atoms and the cell itself, you do not need to apply external pressure - infact, applying pressure will qualitatively change your results. $\endgroup$
    – Xivi76
    Jul 28 '20 at 18:09
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    $\begingroup$ Could you please elaborate it a little? $\endgroup$
    – Alfred
    Jul 28 '20 at 18:11
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    $\begingroup$ Graphene you mean? If you apply pressure, obviously the lattice parameters are going to change. If you do a fixed cell calculation and apply pressure, the atoms themselves are going to feel the extra force. This will affect atomic positions, and in turn the bandstructure and various other properties. $\endgroup$
    – Xivi76
    Jul 28 '20 at 20:16
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One of the possible justification is to know how the material behave under high pressure. You can think in synthesize the material under different thermodynamic conditions or even thinking in some materials you can get from the inner core of the Earth or from meteorites.

A beautiful example recently simulated and them obtained in laboratory is related to what was called by the authors forbidden chemistry1: they simulate the formation of sodium chloride ($\ce{NaCl}$) at high pressures and show how the 1:1 ionic stoichiometry was broken. They not only simulate it but also growth the crystals on the laboratory and characterized by X-Ray diffraction. The new ionic sodium chloride stoichiometry where $\ce{Na3Cl}$, $\ce{Na2Cl}$, $\ce{Na3Cl2}$, $\ce{NaCl3}$, or $\ce{NaCl7}$.

More information about the method and other experimentally confirmed prediction can be seen in the USPEX page.

  1. Jordi Ibáñez Insa. Reformulating Table Salt Under Pressure. Nature, 342, 1459-1460 (2013) (DOI: 10.1126/science.1247699)
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