[QE-users] Phonon Dispersion of SrTiO3, EuTiO3 using Hubbard

[simon rombauer]

Hello Paolo,

thank you very much for your remarks, they gave helped a lot. Using a 
fixed occupation I was able to calculate the dielectric constant. As for 
the phonon dispersion I use ALAMODE and phonopy. As Lorenzo Paulatto 
pointed out, the imaginary frequencies I've got with ph.x in SrTiO3 stem 
from instabilities at 0K.

Thanks again and have a nice weekend everyone,
Simon

Am 2022-06-12 16:14, schrieb Paolo Giannozzi:
> On 12/06/2022 12:00, simon rombauer wrote:
> 
>>      1) Why is it not possible to set epsil = .true., lraman .true. ?
> 
> because the system is (treated as) a metal: you are using a broadening
> 
>>      2) Is it normal that the ph.x code takes a lot of comupting time 
>> compared to pw.x scf, nscf, bands calculations? (ETO phonon at gamma 
>> took over 13 h)
> 
> it depends. Plain (GGA, normconserving) phonon calculations should
> take no more time than (a small factor depending upon the phonon
> wave-vector) * (time needed for a scf calculation) * (number of
> linear-response calculations = 3* number of atoms * number of
> wave-vectors). Ultrasoft phonons are definitely slower. DFT+U don't
> know, might be slower as well. It depends upon the number ot terms
> appearing in the linear-response calculation
> 
>>      3) Almost all phonon frequencies are negative, which indicates 
>> instabilities in the system (?), I've run vc-relax calculations and 
>> used the new lattice parameters but got the same results, any idea 
>> what these negative frequencies could tell me?
> 
> (negative => imaginary actually) You should first of all figure out
> whether they are an artifact of the calculation or not. For sure, your
> threshold for linear-response convergence (tr2_ph) is too large. Have
> you tried a simpler version of a similar system?
> 
>>      4) Is it possible obtain the dielectric function for these 
>> materials in a different way?
> 
> For effective charges you may use the Berry's phase approach
> 
>> I hope you are still around and thank you for reading :)
> 
> if you are referring to the mailing list: yes, it still works, except
> for people using @gmail.com addresses, and for those who don't check
> their spam folder :-(
> 
> Paolo
>> 
>> Have a nice day,
>> 
>> Simon Rombauer
>> Physics student
>> University of Augsburg
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