[QE-users] Strange problem with graphene phonons: convergence makes things worse

[Christoph Wolf]

Dear all,

I am curious if some of you have experience with the following: in normal
LDA/GGA calculations my graphene phonon dispersions always end up with a
small problem at the "M" point of the BZ; the highest optical mode is split
from the lower mode by maybe 50/cm (almost all published papers show this,
for example PRB 78 081406(R) (2008).  In all my calculations the highest
mode "collapses" and the splitting vanishes when I increase the k-grid
and/or q-grid, in this sense the phonon dispersion converges to a final
form but not the "right" one.

Now I know that graphene is tricky to converge but with inputs as in the
above cited paper (k=18x18x1) and a q-grid (9x9x1) the phonon dispersion
looks OK, at k=72x72x1 and q=12x12x1 it looks fairly bad.

Anyone got a similar experience and maybe a solution?

Thanks in advance!
Best,
Chris
-- 
Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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