[Pw_forum] contradictory results of electron-phonon interaction for different k-grids

[Lorenzo Paulatto]

On 12 December 2012 05:42, 高淼 <miaogao at ruc.edu.cn> wrote:

> (3) For the second group, after applid asr, there are lots of imaginary
> frequencies, with the
>
> maximum about -167 cm^-1 at q=(0.186667  0.323316  0.199453). The result
> contradict that of
> the third group. But the first group k-grid show similar behavior with the
> third group,
> why does the second group k-grid show different results?
>
> Dear Miao Gao,
the equilibrium lattice parameter and atomic positions depend on the choice
of functional: you should perform an independent accurate variable-cell
relax for each set of pseudopotentials.  By accurate I mean something like
    forc_conv_thr=1.d-5
    etot_conv_thr=1.d-6
which in turn will require conv_thr of order 1.d-12 or tighter.

High-symmetry materials (i.e. when the atomic sites are Wyckoff positions
with no free parameter) tend to always give "good" phonon dispersions, even
if they are not properly relaxed; when the symmetry is lower even a small
displacement from the equilibrium will give some negative frequency.

bests




-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
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