[Pw_forum] atomic displacements in phonon calculations

[Azadi, Sam]

Dear Paolo, 

Thanks for your answer. 
You pointed out that  the “given displacement pattern” only depends on 
the symmetry of the unperturbed crystal and are chosen along irreducible representations
of the symmetry group. 
Sorry if I’m slow, but I still don’t understand where does the “given displacements  pattern” come from? 
As you said there is a given displacement pattern and then the linear response formalism is employed. 
Do you mean it’s only depend on the symmetry of system? 
If I perform DFPT phonon calculations at the Gamma point for H atoms in BCC lattice and Fe atoms 
arranged on BCC lattice, am I supposed  to see the same “displacement pattern” ? 
I agree that the normal coordinates vector are normalised. 

Thanks in advance. 

Yours, Sam

 

> On 1 Mar 2017, at 12:44, Paolo Giannozzi <p.giannozzi at gmail.com> wrote:
> 
> 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
> -- 
> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
> 
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