[QE-users] unit mismatch between PH generated frequencies and manually generated eigenvalues

[Giovanni Cantele]

Dear Shubham,
I will add a little contribution to the discussion. Prior to it, I would
strongly recommend, as it has been correctly pointed out, that before doing
ANY calculation
you become aware (using tutorials, textbooks and so on) about exactly what
you are calculating, what are the units, and so on. In many cases (but I do
not know if
this is your case!), skipping a necessary initial (self!) training produces
much more waste of time than spending time in understanding how things work.

After inspection of the plots you sent, it seems that the problem you have
is other than a magnitude mismatch. Indeed, a magnitude mismatch is just
a scaling factor, all the bands get multiplied by 10 or 100 or any odd
factor that might be obtained by combinations of Ry = 13.6 eV, bohr radius
and any
other relevant scale unit might apply in this case.

What I simply tried is to superimpose the two phonon band structures and to
stretch one to try to make it overlap with the other. No success in doing
that.
Effectively, you could for example look at the 6th band. The one on the
left is almost flat, the one on the right is not. Moreover, in the left
plot its maximum is
maybe the small dip at the x-axis center, whereas on the right there are
two pronounced maxima around the x-axis extrema. As further proof, the 7-th
band
shares a frequency range with the 6th one on the right, whereas in the left
the two are separated by a small gap.
So, I would conclude that the two band structures do not differ by a
scaling factor and are, for some reason, definitely different! This is also
confirmed by the s
econd image you sent, with three plots. In that case, the right plot was
obtained after multiplying the middle one by a factor and indeed the two
band structures
look identical, apart from the y-axis scale.

How could it happen? Some possible answers:
i) you diagonalized a wrong matrix (for example: did you properly include
the mass factors?)
ii) you are not plotting the same quantity. If I'm not wrong, and there is
not just a scaling factor between left and right, could it be that for
example
when you make the diagonalization you get the squares of the frequencies,
which would explain the NOT UNIFORM stretching of one band structure with
respect to
the other? This or similar things can explain your issue if you
diagonalized the matrix by hand, not of course if the diagonalization was
produced
by a quantum-espresso executable, where units are correctly fixed.

So, the suggestion is to start from scratch, trying to understand, at each
step, what you're doing, which quantity you obtained and with which units.
I'm pretty sure that
it will not be that difficult for you to figure out what's happening!

Giovanni
-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it <giovanni.cantele at spin.cnr.it>
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: https://sites.google.com/view/giovanni-cantele/home


Il giorno mar 8 apr 2025 alle ore 17:03 Shubham Patel <
spatelphy at iitkgp.ac.in> ha scritto:

> Dear developers and users,
>
> I am trying to calculate the frequencies (freqs) using Quantum Espresso
> PHonon calculations as well as using dynamical matrices generated using
> matdyn.x. I find that there is a unit mismatch in the freqs which gives
> rise to a magnitude difference. In the first case, I calculate phonon band
> structure using matdyn.x --> plotband.x. While in the second case I
> generate dynamical matrices using matdyn.x and then diagonalize the
> matrices along the kpath and generated exactly same band dispersion except
> the magnitude on the y-axis (frequency range). Could anyone please tell
> what is this factor which brings the magnitude mismatch? I am also
> attaching a figure to compare.
>
> Thank you very much for any help.
>
> Sincerely,
>
> Shubham_______________________________________________________________________________
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