[Pw_forum] atomic displacements in phonon calculations

[Paolo Giannozzi]

On Wed, Mar 1, 2017 at 1:22 PM, Azadi, Sam <s.azadi at imperial.ac.uk> wrote:

> 3N normal coordinates related to 3*N normal modes are displaced compared to relaxed positions, if I’m right.

Nothing is actually "displaced": what is computed is the linear
response to a given displacement pattern, using perturbation theory.
The amplitude of the perturbation does not affect the calculation.

Specific displacement patterns depend upon the symmetry of the
(unperturbed) crystal and are chosen along irreducible representations
of the symmetry group. I think they are normalized  but I don't
remember exacty how.

Paolo
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Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
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