[Pw_forum] Wild phonons

Miguel Marti­nez miguel.martinez at ehu.es
Wed Dec 19 18:40:52 CET 2007

Dear Nicola,

Thank you very much for your answer. One of the things that puzzled me is 
that k-point convergence was way easier on the NC LDA pseudopotential 
available on the web. Or at least it seemed that way. I always thought that 
including semicore states would only incur in a cutoff "penalty", so to 
speak. Or maybe the semicore states are more sensitive to changes only 
properly descrived in a fine mesh.

Finally, I'd like to apologise everybody for not including my affiliation 
in the signature when using the webmail client.



PS: Calculating elastic constants at finite pressure does indeed require 
knowledge of what one is doing, especially since nearly every single 
textbook assumes zero presssure in its derivations.

Nicola Marzari escribió:
> Dear Miguel,
> is the q=0.1 0.1 0 the result of a direct phonon calculation,
> or the result of an interpolation ? If it's a matdyn interpolation,
> I wouldn't be surprised at all - small changes at q=0 can really affect
> your interpolation.
> If it's a direct calculation, my only comment would be that a change 
> from 0.02 to 0.15 is not that small (with smearing, you should think
> at the inverse - i.e. it's much easier to converge with respect to
> k-points when you have 0.02, that when you have 0.01).
> This doesn't really solve your problems, of course, and the only thing
> you need to figure out is if 26x26x26 and 0.02 is good, or not - and
> the only way is to increase the k-point, and making sure htat things do
> not change, and then decreasing the smearing, and retesting (painful).
> I've seen samplings as high as 40x40x40 to get the elastic constants
> right in Nb, from the sound velocities.
> My experience is that it is less expensive to get elastic constants
> from the energy than form the phonons, but once you really ramp up
> the k-points (as said, even 40x40x40, and you are converged with the
> smearing - not too small, not too large) the results are the same.
> Note that at finite pressure one might need to be careful about
> energy and enthalpy and what is exactly that you are calculating -
> my guess is that you need the second derivatives of the enthalpy, and I
> suspect that is what the phonon slopes would be, but it would be better
> to check carefully the literature. Anyone care to comment ?

Miguel Martínez Canales
    Dto. Física de la Materia Condensada
    Facultad de Ciencia y Tecnología
    Apdo. 644
    48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5326

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