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Re: [abinit-forum] Phonon calculation: negative values of acoustic phonon frequency
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- From: "matthieu verstraete" <matthieu.jean.verstraete@gmail.com>
- To: forum@abinit.org
- Subject: Re: [abinit-forum] Phonon calculation: negative values of acoustic phonon frequency
- Date: Thu, 25 Sep 2008 10:29:11 +0200
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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
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
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
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
- [abinit-forum] Phonon calculation: negative values of acoustic phonon frequency, Thanusit Burinprakhon, 09/24/2008
- Re: [abinit-forum] Phonon calculation: negative values of acoustic phonon frequency, ilukacevic, 09/25/2008
- Re: [abinit-forum] Phonon calculation: negative values of acoustic phonon frequency, matthieu verstraete, 09/25/2008
- Re: [abinit-forum] Phonon calculation: negative values of acoustic phonon frequency, ilukacevic, 09/25/2008
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