[Pw_forum] kp

[meisam.a63 at gmail.com]

Just to add that plain wave is not a good approximation to reproduce molecular orbitals.

Mortaza A

PhD of Physics 
Ulm University, Germany 

> On 28 Nov 2016, at 21:43, Giovanni Cantele <giovanni.cantele at spin.cnr.it> wrote:
> 
> 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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