[Pw_forum] Re: Phonon frequencies different for equivalent q points

[stewart at cnf.cornell.edu]

Hi Eyvaz, 

   For my calculations, I specified the phonon q-vector in the nscf 
calculation in units of (2pi/a).  For both calculations, the k-points were 
calculated using 

K_POINTS {automatic}
20 20 16 0 0 0 

I believe this is the standard way to do these calculations.  Am I missing 
something? 

Regards, 

Derek 

Eyvaz Isaev writes: 

> Hi Derek, 
> 
> Might it be that you used k-points in direct
> coordinates instead of cartesian?  
> 
> Bests,
> Eyvaz. 
> 
> --- stewart at cnf.cornell.edu wrote: 
> 
>> Hi everyone,  
>> 
>>    I have been running some phonon calculations for
>> an hcp Pd system and I 
>> am running into a problem.  
>> 
>> I am looking at the high symmetry lines, Gamma-K-M
>> along the [x,x,0] 
>> direction going from [0,0,0] to [1,1,0]*(2pi/a) and
>> the Gamma-M line along 
>> the [x,0,0] going from [0,0,0] to
>> [1/sqrt(3),0,0]*(2pi/a).  
>> 
>> The phonon frequencies predicted at the M point for
>> each calculation  
>> 
>> [1,1,0]*(2pi/a) and [1/sqrt(3),0,0]*(2pi/a) should
>> be equivalent.  
>> 
>> However, they are fairly different.
>> [1,1,0]  
>> 
>>     omega( 1) =       2.805385 [THz] =     
>> 93.578178 [cm-1]
>>     omega( 2) =       3.004742 [THz] =    
>> 100.228087 [cm-1]
>>     omega( 3) =       4.199464 [THz] =    
>> 140.079956 [cm-1]
>>     omega( 4) =       5.667289 [THz] =    
>> 189.041683 [cm-1]
>>     omega( 5) =       5.820376 [THz] =    
>> 194.148127 [cm-1]
>>     omega( 6) =       6.358473 [THz] =    
>> 212.097247 [cm-1]  
>> 
>> 
>> [1/sqrt(3),0,0]  
>> 
>>     omega( 1) =       3.747138 [THz] =    
>> 124.991899 [cm-1]
>>     omega( 2) =       4.763040 [THz] =    
>> 158.878972 [cm-1]
>>     omega( 3) =       4.797494 [THz] =    
>> 160.028243 [cm-1]
>>     omega( 4) =       5.155421 [THz] =    
>> 171.967476 [cm-1]
>>     omega( 5) =       5.867315 [THz] =    
>> 195.713877 [cm-1]
>>     omega( 6) =       6.220546 [THz] =    
>> 207.496457 [cm-1]  
>> 
>> Now hcp Pd only exists as small films on special
>> substrates.  Normally, Pd 
>> is fcc.  However, the rest of the phonon dispersion
>> curve looks fine (no 
>> imaginary data points).  I am using ecutwfc=44.0 and
>> ecutrho=448 for my scf 
>> calculations with a 20x20x16 k-point grid.  The
>> total stress on my relaxed 
>> system is 0.09 kbar so I don't think that is the
>> issue.  
>> 
>> For the phonon calculations, my input is as follows  
>> 
>> phonons of pd at 1.00
>> &inputph
>>    tr2_ph=1.0d-14,
>>    alpha_mix(1)=0.7,
>>    prefix='pd',
>>    amass(1)=106.42,
>>    fildyn='pd.dyn1.00',
>> /
>> 1.00 1.00 0.00  
>> 
>> I am using espresso-3.0.  Are there any changes in
>> the CVS version that 
>> would affect this calculation?  
>> 
>> Any suggestions or thoughts would be greatly
>> appreciated!  
>> 
>> Thanks,  
>> 
>> Derek  
>> 
>> 
>> ################################
>> Derek Stewart, Ph. D.
>> Scientific Computation Associate
>> 250 Duffield Hall
>> Cornell Nanoscale Facility (CNF)
>> Ithaca, NY 14853
>> stewart (at) cnf.cornell.edu
>> (607) 255-2856  
>> 
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>  
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################################
Derek Stewart, Ph. D.
Scientific Computation Associate
250 Duffield Hall
Cornell Nanoscale Facility (CNF)
Ithaca, NY 14853
stewart (at) cnf.cornell.edu
(607) 255-2856