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 <[email protected]> > 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 <[email protected]> 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. >> _______________________________________________ >> Pw_forum mailing list >> [email protected] >> http://pwscf.org/mailman/listinfo/pw_forum > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum
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