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

[stefano de gironcoli]

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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