[Wien] basis set size for oxygen crystal

[Laurence Marks]

What you find is about consistent with my observations. I would say
that for an RMT of 0.5 a fairly good calculation is roughly an RKMAX
of 3.5, and for an RMT of 2.0 an RKMAX of 8. As a rough guide this
corresponds to (linear approximation)

RKMAX = 3.5 + 3*(RMT_min-0.5)

This is a crude estimate, to be used with extreme caution.

In terms of the physics, my thoughts (Peter probably knows better).
With small RMTs Wien2k can have problems because at the muffin tin
boundary the density is large which can lead to a large discontinuity
of the density in APW+lo. As RKMAX increases the discontinuity
decreases. At least in part this is probably because the smaller the
RMT, the more rapid is the variation in density around the muffin tin
and a larger PW basis set is needed to better match the density
changes within the muffin tin.. There are also subtle issues with the
O linearization energies as the automatic search often fails to find
the optimum energy when the density is not well confined within the
muffin tin, although this is not that critical (I think, fingers
crossed).

N.B., I assume you have taken care of other issues, for instance
running spin-polarized as this is needed for O2, using a larger GMAX
or oversampling which I think is also better for the Coulomb potential
with small RMTs (similar to H)
N.N.B., If you are doing this calculations for thermodynamics, of
course PBE/LDA/WC are pretty bad for O2.

On Wed, Jun 19, 2013 at 8:07 AM, Stefaan Cottenier
<Stefaan.Cottenier at ugent.be> wrote:
>
> Dear wien2k community,
>
> I'm puzzled by the following observation about convergence tests for a
> molecular crystal built entirely from O2-molecules (two O2-molecules per
> unit cell): the basis set needs to be particularly large before even the
> the volume of the unit cell can be reliably determined.
>
> Does anybody knows what is the mathematical/physical reason for this?
>
> More details:
>
> The RMT for oxygen is 1.02, dictated by the bond length in the O2
> molecule. The number of k-points was safely converged. For any basis set
> reported in the table hereafter, the E(V)-curves were nice and smooth --
> only the position of the minimum and the curvature (bulk modulus) keep
> changing.
>
> The table lists the RKMax value, the matrix size, the corresponding
> RKMax if the muffin tin radius would be 2.0 (that gives a better feeling
> of the large size of this basis set) and the volume of the unit cell
> determined from an equation of state fit:
>
> 4.38  574  8.6  487.7
> 4.78  750  9.4  506.4
> 5.20  945 10.2  502.4
> 5.60 1182 11.0  503.7
> 6.00 1426 11.8  505.0
>
> There might be a numerical scatter of about 1-2 in the volumes given in
> the right-most column. This means that the difference between the last
> three volumes is probably negligible, and something as 504 au^3 is the
> converged volume. But the jumps from 487 to 506, and from 506 to 502 are
> definitely not due to numerical noise.
>
> Why does this case require such a large basis set?
>
> Thanks,
> Stefaan
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> SEARCH the MAILING-LIST at:  http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html



-- 
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu 1-847-491-3996
"Research is to see what everybody else has seen, and to think what
nobody else has thought"
Albert Szent-Gyorgi