[Pw_forum] a few frequencies in PHONON calculation

[stefano de gironcoli]

Dear Eduardo,
   I don't know whether the code can perform what you want.
   I think that with some coding one can instruct the code to compute 
only the irreducible representations (all of them) corresponding to a 
given symmetry (for a low symmetry system not much to gain from this, 
I'm afraid).
   Alternatively one can only move a few atoms (in all directions) and 
if the mode that is searched is localized on those atoms a reasonable 
approximation for its frequency can be obtained. This might be 
appropriate to study vibrational modes of a light adsorbate on a heavy 
substrate, and maybe in some other particular situations.
   I don't know how last_irr works but I think it's more a way to break 
a long calculation in many smaller chunks rather than a way to perform 
partial calculations. The code has no way to know which pattern you 
would be interested in.

stefano

On 24/12/2015 12:54, Eduardo Menendez wrote:
>
> Hi everybody,
>
> Let me insist with a question I posted two days ago.
> I interpret from the manual that one can compute a few phonon 
> irreducible representations. If this means that one can compute a few 
> phonon frequencies for a big system that could be useful to do short 
> convergence tests. I guess these frequencies are true for a high 
> symmetry system, but for a low symmetry system I do not know (should I 
> ) how  the irreducible representations can be determined before 
> diagonalizing the full dynamical matrix. I have mad a test using 
> last_irr = 10 , and then last_irr = 14, and I get different 
> frequencies. Maybe I do something wrong, or maybe I just do not 
> understand the goal of "last_irr" keyword. I appreciate that someone 
> kindly clarifies this issue.
>
> Best regards
> Eduardo
>
>
> Date: Tue, 22 Dec 2015 14:48:45 -0300
> Subject: [Pw_forum] a few frequencies in PHONON calculation
> Hi,
> I have one doubt about the PHONON code. Is it possible to compute just 
> a few phonon frequencies using the PHONON keywords last_irr, ldiag ? 
> For example, using
>
> last_irr = 10,
>   ldiag=.true.,,
>
> may one find the frequencies of just the first irreducible 
> representations ?
>
> Thank you,
>
>
> Eduardo Menendez Proupin
> Departamento de Fisica, Facultad de Ciencias, Universidad de Chile
> URL: http://www.gnm.cl/emenendez
>
> “No cometerás actos impuros ni publicarás en revistas open-acces”
>
>
>
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