I would expect there to be slight differences in the energy versus cell for
the two approaches, which could easily lead to forces.

That said, I doubt that the forces will have any effect on what you are
trying to calculate. Do a simple test such as a small TiO2 supercell and
displace an O.

---
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 10:04, "pavel.ondra...@email.cz" <pavel.ondra...@email.cz>
wrote:

>
> ---------- Původní zpráva ----------
> Od: Laurence Marks <laurence.ma...@gmail.com>
> Komu: A Mailing list for WIEN2k users <wien@zeus.theochem.tuwien.ac.at>
> Datum: 29. 1. 2016 13:34:23
> Předmět: Re: [Wien] force difference between LAPW and PAW method
>
> Did you check the absolute cell size for the two - minimize the energy
> w.r.t. size?
>
>
> The absolute cell size was fixed (same in both softwares of course) and
> was calculated using experimental densities for the mixtures. No
> minimization with respect to cell volume was done, however I would expect
> it to be near equilibrium. My main goal with those cells is to calculate
> the XPS shifts of O1s peak (using the slater transition state method) to
> help in evaluation of some experimental XPS data, so I wanted to stick with
> the experimental densities.
>
>
> Would some stress in the cell explain the difference between the methods?
>
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