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["matthieu verstraete" <matthieu.jean.verstraete@gmail.com>]

Hello again,

asr 1 or 2 should be fine. If you still have problems (which ones?) you really need to send us more information about your run and results. If it is just acoustic phonons at gamma, asr 1 or 2 should make them exactly 0.

Matthieu

2008/9/25 GaoTao <gaotaoscu@hotmail.com>

Hi Matthieu,
 
I have the same experience that there are imaginary frequencies in the same input for phonon calculation of AlAs. I do not know how to solve it. According to explanation in mannual of ABINIT, there are 3 options:'0','1' and '2'. After version 5.3, default value 'asr' is set as '1'. Which number would you suggest to set in this calculation? '0' or '2'? Would you like give some recommendation?
 
Thanks in advance.
 
Tao

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Date: Thu, 25 Sep 2008 10:29:11 +0200
From: matthieu.jean.verstraete@gmail.com
To: forum@abinit.org
Subject: Re: [abinit-forum] Phonon calculation: negative values of acoustic phonon frequency

Hello

you are seeing a very habitual feature of pseudopotential calculations of phonon frequencies. There have been quite a few posts to the forum in the past, please check the archives for keywords before posting new ones.

Basically you are breaking the acoustic sum rule (but only by a negligible amount in your 2 cases - less than 1 cm-1) because of the evaluation of the pseudopotential terms on a real space grid: the grid is not strictly translationally invariant, and this gives a small artificial variation of the total energy wrt displacement of atoms (in particular for the acoustic modes at Gamma, which should be 0). This can be compensated by re-inforcing the acoustic sum rule (see *asr* input variables for abinit and anaddb), and in your case this will work fine.

Matthieu

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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Matthieu Verstraete

European Theoretical Spectroscopy Facility (ETSF) 
Dpto. Fisica de Materiales, 
U. del Pais Vasco,            
Centro Joxe Mari Korta, Av. de Tolosa, 72,   Phone: +34-943018393
E-20018 Donostia-San Sebastian, Spain        Fax  : +34-943018390

Mail : matthieu.jean.verstraete@gmail.com
http://www-users.york.ac.uk/~mjv500

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--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Matthieu Verstraete

European Theoretical Spectroscopy Facility (ETSF) 
Dpto. Fisica de Materiales, 
U. del Pais Vasco,            
Centro Joxe Mari Korta, Av. de Tolosa, 72,   Phone: +34-943018393
E-20018 Donostia-San Sebastian, Spain        Fax  : +34-943018390

Mail : matthieu.jean.verstraete@gmail.com
http://www-users.york.ac.uk/~mjv500