[Wien] Converging Gmax

[Peter Blaha]

A larger GMAX improves both, the representation of the Coulomb- and the XC-potential (energy)
in the interstitial. We know that for H-containing systems large GMAX can be essential.

For most other compounds, a GMAX increase is not necessary (although it should never be "wrong"),
because the Coulomb part is usually well described, while for GGAs the XC-part might not.
For those reasons we introduced an IFFT-FACTOR in lapw0, which only affects this term, but not
the Coulomb part. Usually it is enough to converge the XC-part with this factor.

ad 1) A large GMAX is always better.
ad 2) I don't know.
ad 3) Never use "dstart again". Of course one can continue with a larger GMAX (or FFT-factor)
using the previously converged density.

David Tompsett schrieb:
> Dear All,
> 
> I have a query about converging energies with respect to Gmax. I am 
> working on a d-metal metallic system.
> 
> I have been attempting to converge the difference in total energy 
> between a non-magnetic and antiferromagnetic spin configuration. The 
> calculation is done with the GGA using RKmax=9 and RMT's of 2.1 and 
> 8000Kpt in the full Brillouin zone.
> 
> With these parameters (which I tested for convergence) I have then tried 
> to converge the energy difference with respect to Gmax and found some 
> behaviour that I can not understand. All calculations were converged 
> with -ec 0.00001.
> 
> For the non-magnetic calculation the energies go as:
> Gmax        Energy
> 12            -28765.981815
> 14            -28765.984603
> 16            -28765.983740
> 18            -28765.983085
> 20            -28765.982506
> 22            -28765.982092
> 24            -28765.981842
> 26            -28765.981561
> 
> So this indicates large changes of order of 0.01Ry as Gmax is changed. 
> This seems like an extraordinarily high Gmax for convergence
> 
> In contrast for the antiferromagnetic calculation we find little 
> variation as we change Gmax:
> Gmax         Energy
> 12            -28765.982610
> 14            -28765.982595
> 18            -28765.982645
> 26            -28765.982645
> 
>  From this I have a few questions:
> 1) Why would a non-magnetic calculation take such a larger Gmax to 
> converge? Is Gmax=26 reasonable?
> 
> 2) Why might the convergence of the AFM calculation be much faster in Gmax?
> 
> 3) When converging Gmax is it acceptable to reuse the charge density 
> (*.clm*) files from the previous calculation? ie. should I run dstart 
> again before each new value of Gmax?
> 
> Thank you for any help,
> David.
> 

-- 
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Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
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