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If I construct a crystal structure using lattice parameters and atomic coordinates as mentioned in a previous post, what difference does it make if I omit (or enter wrong parameters) for the displacement parameters U or B?

How relevant are these displacement parameters while modelling the material?

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    $\begingroup$ Could you please clarify what the displacement parameters U and B are? It is not clear to me from the linked post. $\endgroup$
    – ProfM
    Commented Jun 10, 2020 at 10:25
  • $\begingroup$ U and B (B = 8π² U) are called atomic displacement parameters. They describe "uncertainty in the position of an atom within a crystal structure. This uncertainty may arise from thermal motion of the atom, leading to variations in the position of the atom between different copies of the unit cell. It may also arise from defects in the observed data." I don't know if any modelling method uses ADPs. $\endgroup$
    – marcin
    Commented Jun 15, 2020 at 15:29

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They can be relevant when comparing a geometry optimization with experimental data: how far the atoms "went"?

(you can use both parameters if you want to constrain the atoms movement during a geometry optimization).

But here we have two issues. The first one is related to the quality of the crystallographic data used to determine both parameters. If using powder x-ray crystallography, the data is least reliable as for single crystal x-ray diffraction.

The second one is that the obtaining of these parameters are valid only for a given, fixed crystal structure but, from the simulations you can get a phase transition where the atoms "move" more than the permitted values of U and B.

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