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

[Eyvaz Isaev]

Hi again,
 
> 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? 

This way  should be OK. I just paid attention that in
your input file for phonons you specified 
(1.0 1.0 0.0)
If this k-point is in your list then it is OK. But I
remember M point's coordinate is (0,1/2,0), or
(1/2,0.2887,0), in units of direct or cartesian
coordinates, respectively. At least, I used this point
for HCP materials (say, Mg or MgB2, and many others)
and I have excellent agreement with experiment.

Bests,
Eyvaz.
 
> 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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> >> 
> >  
> > 
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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 
> 
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> http://www.democritos.it/mailman/listinfo/pw_forum
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