[QE-users] question about wave functions phonon code

[Stefano Baroni]

100 is not equivalent to 110 in FCC … SB

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Stefano Baroni, Trieste -- http://stefano.baroni.me

On 27 Sep 2021, at 19:55, Jacopo Simoni via users <users at lists.quantum-espresso.org> wrote:


Hi thanks for the reply. I need to compute a sort of finite temperature electron-phonon coupling, so I needed to go into the code and I wrote my own routine to do that, I am debugging the code and I noticed that the result at different but equivalent q points, let's say X=(1,0,0) and X=(1,1,0) in FCC are not the same as I should expect.
Then I noticed that the wave function (evq) in reciprocal space differs (that is expected) but I do not understand  the relation between the two, that was why I was asking the previous question. In the meantime I found a mistake in the way I was computing the potential derivative, that is probably the main source of my problem. I was wondering also what does the routine symdyn_munu_new exactly do? Why does the dynamical matrix have to be symmetrized in the basis of the modes ?

Thanks in advance,
Jacopo Simoni, Lawrence Berkeley National Lab.

On Mon, 27 Sept 2021 at 00:20, Lorenzo Paulatto <paulatz at gmail.com<mailto:paulatz at gmail.com>> wrote:
Hello Jacopo,
instead of answering your question, I may ask you what you actually want to do, because chances are that your problem has already been met by others.

E.g. there are ways to eliminate this phase, or to neutralize it to compute the derivative w.r.t. the wave vector. But more often, when you have an observable quantity that comes from a sum over the k-points, the phases will cancel out in the total, if done correctly. It is actually a good way to check that your formulas are correct.

Hth

--
Lorenzo Paulatto

On Sat, Sep 25, 2021, 21:28 Jacopo Simoni via users <users at lists.quantum-espresso.org<mailto:users at lists.quantum-espresso.org>> wrote:
So this means the phase factor is of the form e^{iG(k)r} where G(k) is the translation vector in rec. space such that
G(k)+q+k=q'+k' (inside the first Brillouin zone)
the phase factor is therefore dependent on the k vector and in reciprocal space the wave functions of the two equivalent q points are shifted by the vector G(k) ?
Is there a variable in the code corresponding to this vector G or to the phase factor itself ?


On Sat, 25 Sept 2021 at 03:42, Paolo Giannozzi <p.giannozzi at gmail.com<mailto:p.giannozzi at gmail.com>> wrote:
On Sat, Sep 25, 2021 at 2:00 AM Jacopo Simoni via users <users at lists.quantum-espresso.org<mailto:users at lists.quantum-espresso.org>> wrote:

This wave function appears different from the wave function at an equivalent q point, for instance if I look at evq at q=(0,0,1), this is different from evq at (1,1,0) that are equivalent by translation of a G vector (I am thinking here at a FCC periodic lattice). The two functions just differ by a phase factor or I am missing something ?

They differ by a phase factor; moreover, in the presence of degenerate eigenvalues, you have no guarantee that the eigenvectors in the degenerate subspace are the same. Finally, the ordering of k+G components is not necessarily the same in the two cases

Paolo

Thanks in advance,
Jacopo Simoni, Lawrence Berkeley National Lab.
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Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine, Italy
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