[Wien] k-mesh for CNTs

[Rocquefelte]

Dear Taiiebeh,

K points are needed to describe the solid state effect (periodicity 
...). In other words, you express the wave function of the system using 
Bloch orbitals instead of molecular orbitals (in a cluster calculation).
As a consequence, the dispersion in energy of the states contained in 
the unit cell you have chosen is described using a periodic function in 
which the wave vector k is defined.

If the system you consider is 1D (i.e. 1D infinite chain along a), then 
you need to define a k-mesh along the a* direction of the reciprocal space.
If 2D (lamellar system in (a, b) plane ): k-mesh along a* and b*.
and if 3D: kmesh along all directions.

In other words an isolated CNT is 1D along c, so you will need a k-mesh 
like (1 1 X) with X defining the number of k-points along c* (where your 
band structure will exhibit a dispersion of the energy levels related to 
the sigma and pi states of the CNT).

Concerning the number of k-points, you have to start with a small 
number. Then increase progressively this number and compare the total 
energies of each calculation. In such a way you will choose the smallest 
mesh allowing to access a "converged" total energy (which does not vary 
anymore by increasing the number of k points).

Regards

Xavier

taiiebeh molarooy a écrit :
> Dear wien users,
> Hi, For run scf of CNTs how should we select k-mesh? should we 
> always select 1 k-mesh in b and c directions? why?
> How many k-points should be used? do number of k-point must be low? why?
> Best regards,
>
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