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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