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

Sun, 16 Nov 2014 23:39:39 -0800 

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 
<degir...@sissa.it <mailto:degir...@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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