[Pw_forum] atomic displacements in phonon calculations

[Azadi, Sam]

Dear Paolo, 

Thanks for your reply. 
I found the information I needed in ‘phq_setup’ and other Phonon subroutines
which generate ‘u’ initial displacement pattern which are also available in xml format. 
I should have checked them before posting my question! 
Thanks for your time. 

Yours, Sam
 
> On 1 Mar 2017, at 15:53, Paolo Giannozzi <p.giannozzi at gmail.com> wrote:
> 
> Displacement patterns used by the code depend upon the symmetry (and
> upon an obscure stochastic algorithm that makes them irreproducible)
> only because the linear-response machinery makes usage of symmetry to
> reduce the amount of computation. In principle you might use whatever
> linear combination of atomic displacements spanning all vibrational
> degrees of freedom (3*N for a system containing N atoms); or, simply,
> atom 1 along x,y,z; atom 2 along x,y,z,; etc.
> 
> Displacement patterns are used only to compute the linear response and
> the dynamical matrix. Once you have the dynamical matrix, you can
> forget about them (not that you need to know anything) and compute
> phonon displacement patterns (this is what you need to know about).
> Phonons are not determined uniquely by symmetry, except in special
> cases.
> 
> Paolo
> 
> On Wed, Mar 1, 2017 at 2:54 PM, Azadi, Sam <s.azadi at imperial.ac.uk> wrote:
>> 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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>> 
>> 
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> 
> 
> -- 
> 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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