[Pw_forum] K_POINTS automatic

[Lorenzo Paulatto]

On 3 December 2012 17:34, zahra vatankhah <vatankhah.z at gmail.com> wrote:

> Dear Lorenzo,
> For more detail, I performed the scf run for graphene/h-BN bilayer with
> both 001 and 111. The Fermi energy was 1.4072 and 1.2002 eV for 111 and 001
> respectively. Then I plot the DOS for both and I set the Fermi to zero, but
> they were different. For 111, the Fermi is in the middle of the small gap
> whereas the fermi crosses the states for 001.( The attached picture is the
> total DOS with 001 without setting fermi to zero, the red line is the fermi
> obtained from scf run with 001 and the green line is the fermi with
> 111.(the correct fermi should be in the middl of the gap)).
>

Dear Zahra,
it looks like the difference is very small, especially sine you have a gap.
Keep in mind that, if you have a gap, in the limit of zero smearing the
Fermi energy can  go anywhere in the gap. This is just a complicated way to
say that there is no Fermi energy in insulators.

On the other hand, both your calculations are wrong; if you want to
simulate non-interacting planes - separated by some vacuum along Z - than
you cannot shift the kpoints along Z. Be sure to have well understood this
point before continuing, as it really is important. Check on some solid
state book how the periodicity of wavefunctions is related to their
k-point, and what kind of periodicity do you expect for your system
(included the artificial periodicity along Z). By setting the shift along Z
you force the wavefunctions from two artificial replicas of the graphene
bilayer to hybridize with each other, which is an unphysical effect.

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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