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.ondra...@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 > _______________________________________________ > Wien mailing list > Wien@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 >

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