[Pw_forum] kp

[ashkan shekaari]

Dear Giovanni,
Thank you so much for your explanatory response. So, as I understood, there
is no way to calculate band structure for a molecule and this is
nonsensical. Is it true?

On Nov 29, 2016 1:46 AM, <meisam.a63 at gmail.com> wrote:

> 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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