[Wien] force difference between LAPW and PAW method

[Pavel Ondračka]

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