[Pw_forum] about Phonon results

[Eduardo Ariel Menendez Proupin]

This happens because
a) 0 frecuencies at Gamma occurs because the energy is is invariant with
respect to arbitrary translations of the whole crystal (the same translation
for all atoms). From this assumption the accoustic sum rule (ASR) is
obtained theoretically.
b) The plane wave DFT calculations do not have this invariance exactly,
because of the plane wave cutoff. The energy is invariant not for arbitrary
translations, but for translations in the FFT grid.
c) I guess that the absolute translation invariance is lost in the back and
forth FFTs of the exchange-correlation potential, which is non linear, and
probably in some part related with the ultrasoft pseudopotentials. Someone
correct me if I am wrong, please.

You may increase the density cutoff (ecutrho) and see if the first three
frequencies get closer to zero. First do it with one of the ESPRESSO
examples, that run in minutes. I guess that I made that test some years ago.

Using larger cutoffs increase a lot the calculation time. That is why you
have the tricky option to impose the ASR in postprocessing (matdyn.x).

Practical way. If your "wrong" frecuencies are smaller than 50 cm^-1, this
is normal. Apply the ASR in the input for matdyn. Check that the other
frequencies do not change more than 1 or 2 cm^-1. Then, if  this makes you
happy, go to write your report

>I have another question about the output: What is the >difference between
the total magnetic moment and the >absolute magnetic moment.
This has been asked in the forum before. Please, google for this. If I
remember well, the absolute magnetic moment is the integral of the absolute
value of the magnetization.

Regards
Eduardo Menendez

On Sat, Jun 14, 2008 at 2:57 AM, <xwzhang at uestc.edu.cn> wrote:

> Dear my pwscf friends,
>
> Recently, I am trying to do phonon calculations in Heusler. As a first
> step, I tried to reproduce the results of a paper by Zayak(PRB, 72, 054113,
> 2005). I choose Ni2MnGe as an example. To my surprise, the acoustic branches
> at Gamma point gives a frequency of about 1 THz, which should be zero. Other
> branches agree with the published data.
>
> I wonder what might be the reason? I attached my scf, and phonon input
> files, and the phonon out put files.
>
> I have another question about the output: What is the difference between
> the total magnetic moment and the absolute magnetic moment.
>
> Thank you and wish you a happy weekend!
>
> Wenxu Zhang
>
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