[Pw_forum] Fourier interpolation

[HaoPan]

Dear Paolo, 

The re-generated phonon from the force constant matrix are negative. 
  0.177215  0.000000  0.000000
  -45.4648  -45.4648  119.2232  626.1196  626.1196  645.7030

comparing to the frequency got by ph.x 

   q = (    0.177215000   0.000000000   0.000000000 ) 

 **************************************************************************
     omega( 1) =       0.431524 [THz] =      14.394177 [cm-1]
 (  0.000000  0.000000  0.129619 -0.022092  0.597312 -0.371775 ) 
 (  0.000000  0.000000  0.118762  0.048530  0.686479  0.000000 ) 
     omega( 2) =       0.431524 [THz] =      14.394177 [cm-1]
 ( -0.000000  0.000000 -0.693560  0.118207  0.111631 -0.069481 ) 
 ( -0.000000 -0.000000 -0.635471 -0.259673  0.128295 -0.000000 ) 
     omega( 3) =       3.535384 [THz] =     117.928499 [cm-1]
 ( -0.661130 -0.445106  0.000000  0.000000  0.000000 -0.000000 ) 
 ( -0.247111 -0.551111  0.000000  0.000000  0.000000  0.000000 ) 
     omega( 4) =      18.715499 [THz] =     624.285984 [cm-1]
 ( -0.000000 -0.000000  0.000000 -0.000000 -0.106238  0.066124 ) 
 (  0.000000  0.000000 -0.000000 -0.000000  0.992140  0.000000 ) 
     omega( 5) =      18.715499 [THz] =     624.285984 [cm-1]
 ( -0.000000 -0.000000 -0.110727  0.058294  0.000000 -0.000000 ) 
 (  0.000000  0.000000  0.989559  0.071511  0.000000 -0.000000 ) 
     omega( 6) =      19.507679 [THz] =     650.710441 [cm-1]
 (  0.080574  0.054886 -0.000000 -0.000000 -0.000000 -0.000000 ) 
 ( -0.402244 -0.910327  0.000000  0.000000  0.000000  0.000000 ) 
 **************************************************************************

Maybe something wrong in the generation of interatomic force constant, but how to check it? Thanks a lot.

Sincerely,
Pan Hao 
Tulane University 







  
Quoting HaoPan <adehaopan at hotmail.com>:

> On each q point I got, the phone frequencies are positive. But after 
> the Fourier interpolation process, There exist negative frequencies.

hard to say what is happening. When you re-generate phonon from
the force constant matrix, you should find exactly the same phonon
frequencies for the wave-vector grid used to generate force constants
in real space. If not, something has gone wrong in the generation
of interatomic force constants

Paolo
 		 	   		  
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