[Pw_forum] atomic displacements in phonon calculations

[Paolo Giannozzi]

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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-- 
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