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## Phonon calculations

Moderators: gmatteo, bruneval, rigna

saikrishna
Posts: 15
Joined: Fri Jun 21, 2019 12:11 pm

### Phonon calculations

Hello
I am doing tutorials trf1_3.in and I am stuck in the place where they calculate the dynamical matrices for phonons. Can someone help me out with that?

Thank you in advance.

ebousquet
Posts: 372
Joined: Tue Apr 19, 2011 11:13 am
Location: University of Liege, Belgium

### Re: Phonon calculations

Dear saikrishna,
What do you mean by "stuck", I mean what is blocking you exactly?
Cheers,
Eric

saikrishna
Posts: 15
Joined: Fri Jun 21, 2019 12:11 pm

### Re: Phonon calculations

Hey Eric
I found the answer for my previous question but I have a doubt on what is a q point in brillouin zone?

Sai krishna.

saikrishna
Posts: 15
Joined: Fri Jun 21, 2019 12:11 pm

### Re: Phonon calculations

Hey
I have a doubt here in executing the file which I wrote. Please tell me my mistake and the correct me asap.
PFA.
Sai krishna
Attachments
phonon3.in

saikrishna
Posts: 15
Joined: Fri Jun 21, 2019 12:11 pm

### Re: Phonon calculations

Hey everybody,
I am running phonon calculations for a material for which I obtained negative frequencies. May I know what does it mean or is it something wrong in the code which is responsible?

Sai krishna N.

ebousquet
Posts: 372
Joined: Tue Apr 19, 2011 11:13 am
Location: University of Liege, Belgium

### Re: Phonon calculations

Dear Sai krishna,
Negative frequencies corresponds actually to imaginary frequencies.
1) These values can appears if your system is not stable w.r.t. atomic displacements (unstable phonon), meaning your structure is not the ground state one. You can find the ground state structure by displacing the atoms following the eigenvector given by the unstable mode(s) (also called mode condensation). Then you know your structure is stable once no "negative" frequency is present in your calculation. In this case these negative frequencies are physically relevant.
2) Your system is supposed to be stable locally or supposed to be the ground state and you still find negative values for phonon frequencies. Here two source cases can be discussed:
a) Your calculation gives the so called "acoustic sum rule" (ASR) problem (the translation invariance is broken in your calculation, which is revealed by missing 3 phonons at zero frequency exactly). You can fix that after your Abinit calculation by using the post-processing tool anaddb and re-enforce the ASR by using the flag asr https://docs.abinit.org/variables/anaddb/#asr.
b) if after imposing the asr it is still a problem then there is probably a problem of convergence of your calculation.
Note that the ASR can be violated a bit a by few cm-1, this still gives a good result for the phonons.
ASR can be imposed directly in the abinit DFPT calculation (https://docs.abinit.org/variables/eph/#asr) but if an ASR violation is present you will not notice it, which can be usefull information; I advise to impose it after DFPT calculation with anaddb for example.
Best wishes,
Eric

saikrishna
Posts: 15
Joined: Fri Jun 21, 2019 12:11 pm

### Re: Phonon calculations

Hey Eric,
Thank you for your reply and now I have a doubt with the code which I have attached and so please help me in solving the issue.
Attachments
phmat2.in
phmat4.in