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

[Lorenzo Paulatto]

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 <mailto: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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-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
+33 (0)1 44 275 084 / skype: paulatz
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