[Wien] force difference between LAPW and PAW method

[Laurence Marks]

Did you check for lattice parameter changes? A relatively small change for
a single unit cell can translate to something larger. Unfortunately you
need a reference compound, which is not so simple in your case. Perhaps
test SiO2, TiO2 & a few simple similar compounds with the PAW & Wien2k.

---
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
http://www.numis.northwestern.edu
Corrosion in 4D http://MURI4D.numis.northwestern.edu
Co-Editor, Acta Cryst A
"Research is to see what everybody else has seen, and to think what nobody
else has thought"
Albert Szent-Gyorgi
On Jan 29, 2016 03:50, "Pavel Ondračka" <pavel.ondracka at email.cz> wrote:

> Dear Wien2k mailing list,
>
> I've been doing some calculation on a moderately large (around 100
> atoms) amorphous-like SixTi1-xO2 cells produced by simulated annealing
> MD in a PAW software. According to my colleague who did the final force
> relaxation, the residual forces on atoms should be around 0.5
> mRyd/Bohr. However when I calculate forces in Wien2k I see forces of
> order of magnitude larger, eg. some as large as 30mRyd/Bohr.
>
> This are exactly the same calculations: both using PBE, same k-grid,
> same RKMAX (calculated from cutoff using the formula from UG) and I've
> spent a lot of time checking the force convergence with respect to all
> possible parameters.
>
> We did some tests with small cells (eg. 10 atoms) and there we can get
> a consistent results with max difference of forces around 10-20% which
> I find reasonable. I'm actually quite clueless about it. I've been
> suspecting user error at the beginning however I haven't found any so
> far. My current theory is that in this large cases without symmetry
> maybe the small differences between methods somehow sum up and hence
> the big difference in final forces, however I would like to hear your
> opinion in this matter.
>
> Best regards
> Pavel Ondračka
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