[Pw_forum] difference in output from dynmat.x and matdyn.x

[Sridhar Sadasivam]

Dear Lorenzo,
I apologize for getting back so late on this. I had posted a reply but
looks like the email bounced back due to some size restriction on the
attachment. But to answer your question, I did not find a negative
frequency anywhere else in the q-point grid. I am attaching my scf, ph, q2r
and matdyn input files. I am facing the same problem with another crystal
structure of SiO2 as well (cristobalite). I feel like I am making some
mistake in the input files that is probably important for polar materials.

I would really appreciate it if someone could look into it. Thanks!
Sridhar
Purdue University

On Mon, Nov 17, 2014 at 2:34 AM, Lorenzo Paulatto <
lorenzo.paulatto at impmc.upmc.fr> wrote:

>  Dear Sridhar,
> normally the sum rules imposed by matdyn are enough to set the acoustic
> bands at gamma to exactly zero. If this is not the case, it normally means
> that you have some negative frequency somewhere else in the q-point grid.
> Please check the other matdyn files to see if this is the case: the
> frequencies are at the end of the file.
>
> kind regards
>
>
>
> On 17/11/14 02:03, Sridhar Sadasivam wrote:
>
> Dear Stefano,
>
>  Thanks for your reply. Here are the frequencies at gamma point when I
> set zasr and asr to 'simple'
>    -12.8981  -12.8981   -4.5998  142.6073  168.0267  168.0267
>   249.0588  249.0588  340.4760  344.4668  379.2589  379.2589
>   417.0274  417.0274  429.6273  457.2113  657.8050  657.8050
>   746.0641  755.8553  755.8553 1032.5200 1032.5200 1037.0491
>  1048.5362 1136.7760 1136.7760
>
>  Thanks,
> Sridhar
>
> On Sun, Nov 16, 2014 at 5:25 AM, stefano de gironcoli <degironc at sissa.it>
> wrote:
>
>>  dear Sridhar Sadasivam
>>       can you try to set zasr and asr to 'simple' in the q2r+matdyn case
>> and report the results ?
>>       thanks
>> stefano
>>
>>
>>
>> On 11/15/2014 11:49 PM, Sridhar Sadasivam wrote:
>>
>>  Dear espresso users and developers,
>>
>>  I am trying to obtain the phonon dispersion of alpha-quartz (SiO2)
>> which has a 9 atom unit cell. I ran ph.x on a 3 x 3 x 3 q-point grid. I am
>> seeing some strange behavior where there seems to be a difference in the
>> frequencies output from dynmat.x and matdyn.x. I will explain my problem
>> below.
>>
>>  I diagonalized the dynamical matrix at the gamma point using dynmat.x
>> with acoustic sum rule set to crsytal. Since SiO2 is a polar material,
>> LO-TO splitting will need to be considered at the gamma point but I am
>> ignoring it currently (I did not specify any direction for LO-TO splitting
>> in the dynmat.x input). This gave me the following 27 frequencies (in
>> cm^-1) at the gamma point: (0,0,0, 152.24, 175.57, 175.57, 256.32, 256.32,
>> 341.09, 354.41, 376.86, 376.86, 419.01, 419.01, 435.37, 455.70, 659.12,
>> 659.12, 744.54, 754.62, 1034.38, 1034.38, 1040.20, 1051.12, 1139.60,
>> 1139.60)
>>
>>  I also used q2r.x (with zasr = 'crystal') to obtain the force constants
>> and then used matdyn.x (with asr = 'crystal') to obtain the frequencies at
>> the gamma point. However with matdyn.x, I obtain the following frequencies
>> at the gamma point: (-29.4088,-22.2535,-22.2535,152.2412,175.8141,175.8141,
>> 256.3373, 256.3373, 341.0892, 355.4227, 376.0495 , 376.0495, 418.6344,
>> 418.6344, 435.3710, 453.6862, 659.7607, 659.7607, 746.4843, 754.8867,
>> 754.8867, 1034.1751, 1034.1751, 1039.7590, 1051.1207, 1139.8191, 1139.8191)
>>
>>  Could someone please explain why I am getting imaginary frequencies
>> from matdyn.x while dynmat.x gives real frequencies at the same q-point
>> (gamma in this case)?
>>
>>  Thanks,
>> Sridhar
>> Purdue University
>>
>>
>>
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>
>
>
> _______________________________________________
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>
> --
> Dr. Lorenzo Paulatto
> IdR @ IMPMC -- CNRS & Université Paris 6
> +33 (0)1 44 275 084 / skype: paulatzhttp://www-int.impmc.upmc.fr/~paulatto/
> 23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
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