[Wien] K-mesh compatibility

[Peter Blaha]

> My question is about k-mesh compatibility. I am doing slab calculations

Unfortunately there is (to my knowledge) no way to make those k-meshes
really equivalent. (how should this work if your lattice changes by
sqrt(3) ??)

In addition to the "k-mesh error", you may get another uncertainty when
doing GGA calculations, since the gradients of rho for the GGA are
calculated numerically on a FFT grid. And this grid is of course quite
different when you change the lattice (angles or length by sqrt(2 or 3)).
Maybe a very large GMAX (you should have this for H anyway) helps to
improve with this).

Regards

> of H on Pd(111) surface using different unit cells. Since some H
> configurations can be calculated in different cells, I can check that
> the different unit cells lead to the same adsorption energies. On that
> respect, it is very important to use equivalent k-meshes in all unit
> cells. For example:
>
> 1x1 hexagonal cell ..................  12x12x1 K-mesh
> 2x2 hexagonal cell ..................  6x6x1 K-mesh
> 3x3 hexagonal cell ..................  4x4x1 K-mesh
> 2x3 hexagonal cell ................... 6x4x1 K-mesh
>
> The former meshes would give exactly equivalent irreducible k-points in
> all cells, independently of the cell size. But what about if I want to
> use a sqrt(3)xsqrt(3) unit cell,  or rectangular unit cells?
> They are no longer integer multiples of the 1x1 cell, so that the
> k-meshes are not exactly compatible. When doing calculations on such
> cells (sqrt(3)xsqrt(3) or rectangular) I get adsorption energy
> differences for the same H configurations in the order of 10-15 meV.
> Such differences are very small, but for system like mine involving H,
> they are unacceptable.
>
> Is there a way to produce exactly compatible k-meshes in different unit
> cells (like 3x3 and sqrt(3)xsqrt(3) for example)?.
>
> Please, notice that this problem has nothing to do with k-point
> convergence (as long as one is working in the convergence region). From
> compatible k-meshes in different cells, I would expect adsorption energy
> differences within 1 meV for the same configurations (not 10-15 meV).
>
> Thank you in advance,
>
>
> Regards
>
>
> --------------------------------------------------------
>
> Cesar Lazo				lazo at fhi-berlin.mpg.de
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                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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