[Pw_forum] comparison of ph.x and dynmat.x results

[Elie M]

Yes, I thought that the non zero negative frequencies that remained still signal instability. Will do what you suggested. Thanks

Elie

> Date: Sat, 8 Sep 2012 09:17:05 +0200
> From: degironc at sissa.it
> To: pw_forum at pwscf.org
> Subject: Re: [Pw_forum] comparison of ph.x and dynmat.x results
> 
> negative (imaginary) frequencies signal instabilities.
> acoustic modes (i.e. rigid global translations on the crystal) at  
> gamma should always have zero frequencies but for numerical reasons  
> they can result in small positive or negative values that can be fixed  
> by the acoustic sum rule.
> 
> in your case the modee at -49, 50 and 73 are the acoustic modes that  
> vanish after ASR inclusion.
> the modes around -370 are other modes and they are unstable..
> 
> move the atoms of your structure along one of this modes and relax it again.
> this will probably break a symmetry that prevented your system to  
> reach complete relaxation
> 
> stefano
> 
> 
> Quoting Elie M <elie.moujaes at hotmail.co.uk>:
> 
> > Dear all, I have done phonon calculations at the Gamma point to find  
> > the vibrational frequencies of a system I am working on and I got  
> > three negative frequencies; the results are:
> >  q = (    0.000000000   0.000000000   0.000000000 )
> >   
> > **************************************************************************    
> >   omega( 1) =     -11.303890 [THz] =    -377.057178 [cm-1]      
> > omega( 2) =     -11.228798 [THz] =    -374.552397 [cm-1]     omega(  
> > 3) =      -1.493780 [THz] =     -49.827127 [cm-1]     omega( 4) =     
> >    1.499866 [THz] =      50.030148 [cm-1]     omega( 5) =        
> > 2.192955 [THz] =      73.149113 [cm-1]     omega( 6) =       
> > 11.690342 [THz] =     389.947822 [cm-1]     omega( 7) =       
> > 15.929343 [THz] =     531.345701 [cm-1]     omega( 8) =       
> > 17.762801 [THz] =     592.503255 [cm-1]     omega( 9) =       
> > 17.814756 [THz] =     594.236307 [cm-1]     omega(10) =       
> > 22.128875 [THz] =     738.139807 [cm-1]     omega(11) =       
> > 24.754227 [THz] =     825.712121 [cm-1]     omega(12) =       
> > 25.174421 [THz] =     839.728307 [cm-1]     omega(13) =       
> > 25.229402 [THz] =     841.562251 [cm-1]     omega(14) =       
> > 31.677488 [THz] =    1056.647257 [cm-1]     omega(15) =       
> > 32.931458 [THz] =    1098.475192 [cm-1]     omega(16) =       
> > 32.974208 [THz] =    1099.901170 [cm-1]     omega(17) =       
> > 37.529033 [THz] =    1251.833794 [cm-1]     omega(18) =       
> > 37.585396 [THz] =    1253.713860 [cm-1]     omega(19) =       
> > 38.689108 [THz] =    1290.529726 [cm-1]     omega(20) =       
> > 44.468725 [THz] =    1483.317012 [cm-1]     omega(21) =       
> > 44.490793 [THz] =    1484.053106 [cm-1]     omega(22) =      
> > 100.618488 [THz] =    3356.271501 [cm-1]     omega(23) =      
> > 100.705119 [THz] =    3359.161186 [cm-1]     omega(24) =      
> > 103.337467 [THz] =    3446.966862 [cm-1]
> > To check whether the first three freqnecies are the accoustic ones  
> > and not instabilities  i applied dynmat.x with asr='crystal' and got:
> >   mode   [cm-1]     [THz]       IR    1   -377.06  -11.3039     
> > 0.0000    2   -374.55  -11.2287    0.0000    3      0.00    0.0000    
> >  0.0000    4      0.00    0.0000    0.0000    5      0.00    0.0000   
> >   0.0000    6    406.11   12.1749    0.0000    7    531.35   15.9293  
> >    0.0000    8    592.50   17.7628    0.0000    9    594.24    
> > 17.8148    0.0000   10    738.13   22.1286    0.0000   11    828.85   
> >  24.8482    0.0000   12    839.68   25.1730    0.0000   13    841.62  
> >   25.2311    0.0000   14   1056.65   31.6775    0.0000   15    
> > 1099.09   32.9498    0.0000   16   1099.25   32.9547    0.0000   17   
> >  1251.32   37.5135    0.0000   18   1253.47   37.5781    0.0000   19  
> >   1290.53   38.6891    0.0000   20   1483.05   44.4608    0.0000    
> > 21   1485.30   44.5283    0.0000   22   3356.09  100.6131    0.0000   
> >  23   3359.58  100.7178    0.0000   24   3446.96  103.3373    0.0000
> > As it is seen, the freqencies are very close but the thing and the  
> > system is stable! Howevere,I could not understand is about the first  
> > 5 frequencies: i still got the first two negative but in addition i  
> > have 3 more zero frequencies; does it mean we have five accoustic  
> > modes due to the symmetry of this particular system?
> >
> > Thanks in advance
> > Elie KoujaesUniversity of NottsNG7 2RDUK
> 
> 
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