[Pw_forum] Re: Phonon frequencies different for equivalent q points
Eyvaz Isaev
eyvaz_isaev at yahoo.com
Tue Apr 11 22:52:17 CEST 2006
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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