[Pw_forum] phonon errors (vtmtrinh at caltech.edu)

[vtmtrinh at caltech.edu]

Dear Prof. Isaev,

Thank you very much for your detailed answer.  It is clear for me now.

MyTrinh


>
> Hi,
>
> --- On Thu, 10/8/09, vtmtrinh at caltech.edu <vtmtrinh at caltech.edu> wrote:
>
>> My main purpose is to calculate the phonon dispersion curve from Gamma
>> to X  for Si. I'd like to reproduce the Si phonon dispersion curve from
>> > Gamma  to X. 
>
> In order to do this one correctly first you should calculate dynamical
> matrices for a given q-mesh, then perform FFT by q2r.x, then apply
> matdyn.x to calculate phonons along  given directions (specifying a list
> of q-points). In this way you apply ASR for both q2r.x and matdyn.x
> calculations.
>
> If you have only \Gamma-point phonons you can use dynmat.x to apply ASR.
>
>> I thought that after the scf calculation for 10x10x10, I can
>> run phonon calculation directly for a series of q points from Gamma to
>> X (let say 5 points).  Then from there, I use q2r.x and then
>> dynmat.x to calculate  intermediate points in order to fix the non-zero
>> > frequency  at Gamma point  (applying ASR) and to get a smooth
>> dispersion > curve  curve. 
>
> This is a wrong way to calculate phonons. Of course, you can calculate
> phonons for any q-point you like, but they are not oblidged to form a
> proper mesh for Fast-Fourier-Transformation. In this way you always  will
> meet troubles with q2r.x (FFT).
>
>> However, the phonon calculation (the first crash) crashed all the
>> >times.  Now I am  testing by running phonon calculation for a mesh of
>> >4x4x4  to see if it  still crashes. If it works, this means that direct
>> calculation of phonon  by specifying directly q points along G-X does
>> not
>> work.  However, this  will be expensive calculation if to get phonon
>> dispersion a long one  direction, I have to do the 3D-messh phonon
>> calculation  first.
>>
>> If I did only a single q-point calculation one at a time,
>> which would be  much faster than does the approach above,
>
> Sorry, I  am not sure I exactly understand what you mean, but calculations
> performed in the right way (see above) give you Interatomic Force
> Constants (IFC) matrix which allows you to evaluate many thermodynamical
> properties, besides  phonons along high symmetry directions.
>
>>how could I  specify q-grid in  q2r.x?
>
> You should not specify q-grid in q2r.x's input file. It (q2r.x) reads
> Sysname.dyn0 file generated by ph.x which contains info about the q-grid
> (divisions along each basis vectors, total number of q-points and q-points
> itself.
>
> Of course, you could calculate dynamical matrices in the old manner,
> specifying a list of q-points (but they have to be a result of KxLxM mesh,
> and calculated before you start phonon calculations). In this case you can
> follow your example given below.
>
>>
>> Also, I have another question, if I would like to apply ASR
>> to fix the phonon freq. at Gamma point, is it correct to just specify
>> the q-grid in  the input files for q2r.x and matdyn.x as follows:
>>
>> &input
>>     fildyn='si.dyn', zasr='crystal',
>> flfrc='si.fc'
>>   /
>>  1 1 1
>>  1
>>  si.dyn1    !this the *.dyn file at gamma
>
> I do not think so, as q2r.x calculates IFC Matrix, not phonon modes as
> matdyn.x does this job.
> How to apply ASR for Gamma-point phonons  see above.
>
> Bests,
> Eyvaz.
>
> -------------------------------------------------------------------
> Prof. Eyvaz Isaev,
> Theoretical Physics Department, Moscow State Institute of Steel & Alloys,
> Russia,
> Department of Physics, Chemistry, and Biology (IFM), Linkoping University,
> Sweden
> Condensed Matter Theory Group, Uppsala University, Sweden
> Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com
>
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