[Pw_forum] kp

[Giovanni Cantele]

if you try to ask the question: "what the k-point mesh is for?" you can easily understand
that a k-point mesh is for the Brillouin zone (BZ) sampling, that is, perform sums over k-points
that approximate, with a required level of accuracy, the integral over the whole BZ. This is
because in a crystal, the energy E(K) depends on the k vectors, and the same holds for the
wave functions. However, when you build for example a cubic supercell including vacuum to
simulate a molecule, you are actually dealing with a fictitious cubic crystal where the repeated
units are sufficiently far apart from each other to not interact.

Absence of interaction means flat (atomic-like) energy levels, that do not have dependence
on the k vector. That is, the E(K) dispersion that you get in the case of a molecule, provided
you include a sufficient vacuum space in your supercell, does not depend on k. If this is
the case, you can use only the gamma point (a larger grid means large but useless
computational time, because you are summing over eigenvalues at different k-points that 
are all the same!) for self consistency and BZ path is not needed, because if you select any
path you should get flat, k-independent bands

Giovanni


> On 28 Nov 2016, at 18:12, ashkan shekaari <shekaari at gmail.com> wrote:
> 
> Dear experts,
> 
> Is it necessary using kpoint mesh when we are dealing with molecules?
> 
> In the case of a molecule in cubic vacuum, what path in BZ should be selected?
> --
> Regards,
> Ashkan Shekaari
> Plasma Physics Research Center
> Science and Research Branch
> I A U, 14778-93855 Tehran, Iran.
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