[Pw_forum] Na phonon issues

[Miguel Martínez Canales]

Dear Nicola,

Thank you very much for your detailed answer, I am very grateful indeed. I 
didn't know there would be such issues outside gamma, and was further 
mislead by the fact that the other pseudopotentials seemed to give a 
similar and sensible answer.

Nicola Marzari escribió:
> The only solution is to calculate the dynamical matrix explicitly on a
> homogeneous q grid that is not too fine (so that the q closest to gamma
> but not gamma has all frequencies well converged), then transform in
> real space, impose acoustic sum rules, and then obtain the phonon
> dispersions everywhere from the fourier transform and diagonalization of 
> the interatomic force constants.

What would one do in case long range effects are not negligible? For 
example, fcc Li at 30 GPa has a soft phonon in the \Gamma-K branch. 
Reproducing it via the IFC matrix seems to require a not so coarse q point 
grid. What would one do? Calculate the elastic constants from classic 
lattice distortions? Introduce phonons from the soft region to include in 
the q2r.x list? Forget about getting accurate or even physical results with 
matdyn?

> One additional note about worst case scenarios: when two dispersions are 
> degenerate, but maybe numerically slightly different, we found an 
> instability in the back-transform (I think because diagonalization and 
> ft do not commute) with the two degenerate branches being degenerate at 
> the original q's, and mostly everywhere, but with one of the two
> displaying a infinite derivative at a critical q close to gamma, in 
> between gamma and one of the original q's. This was the case for CNTs.

Oh, well, this gives another meaning to "treacherous computing". Thanks 
again for your kind answer,

Miguel

PS: Increasing the degauss only yielded negligible differences in the 
phonons at 0 1/10 1/10.

-- 
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Miguel Martínez Canales
Dto. Física de la Materia Condensada
UPV/EHU
Facultad de Ciencia y Tecnología
Apdo. 644
48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5437
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