[Pw_forum] Phonon frequencies different for equivalent q points

[Eyvaz Isaev]

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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