[Wien] force difference between LAPW and PAW method

[Peter Blaha]

This looks fairly large.

I do NOT understand your statement of using "the same RKMAX" ???
PAW does not have RKMAX and their KMAX has NOTHING to do with ours.
What was your RKMAX and what are youre sphere sizes ???

k-mesh: not only the grid is important ! As far as I know, VASP usually 
uses a "huge" temperature broadening. What FERMI-method are you using ?

spin-polarization ??

Is the PAW done with VASP and a fairly new release (accurate PAW 
potentials ?)

On 01/29/2016 10:50 AM, Pavel Ondračka 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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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
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