[Wien] force difference between LAPW and PAW method

Peter Blaha pblaha at theochem.tuwien.ac.at
Fri Jan 29 12:56:00 CET 2016


Maybe one should use in VASP also a PAW potential which has Si-2p as 
valence.

On 01/29/2016 12:47 PM, pavel.ondracka at email.cz wrote:
>
> ---------- Původní zpráva ----------
> Od: Peter Blaha <pblaha at theochem.tuwien.ac.at>
> Komu: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at>
> Datum: 29. 1. 2016 12:18:58
> Předmět: Re: [Wien] force difference between LAPW and PAW method
>
>
>     If the disordered compound goes metallic ??? it could be a problem. The
>     default WIEN2k is using TETRA, while VASP uses a large broadening.
>
>
> The resulting structure has gap at least as large as TiO2.
>
>
>     Again, forget the 400 eV of the PAW calculation. It has NOTHING to do
>     with our RKmax and our basis sets are usually more efficient.
>
>     In any case, RKMAX=8.5 is "more than overconverged", since your
>     smallest
>     sphere is probably Oxygen. I'd start such a calculation with RKMAX=5
>     and
>     later check with 6 and/or 7.
>
>
> True, for usual calculations I use value RKMAX around 7.0 for such
> compounds, I went to 8.0 just for convergence checking purposes. However
> average differences in forces between 7.0 and 8.0 were quite small <
> 0.2mRyd/Bohr
>
>
>     Another possible problem: SiO2 might have small spheres for Si leading
>     to quite some core leakage ??? Either use .lcore or put the Si-2p as
>     semicore !
>
>
> Indeed I've noticed that silicon gets very small sphere especially with
> the new scheme in setrm. I'm using the "original" scheme, where the Si
> usually gets same sphere as O, around 1.5. Also Si2-p states were chosen
> as semicore. No core leakage warnings in logs.
>
>
>     On 01/29/2016 12:04 PM, Pavel Ondračka wrote:
>      > Peter Blaha píše v Pá 29. 01. 2016 v 11:43 +0100:
>      >> 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 ???
>      >
>      > Sorry, the right word should probably be "equivalent RKMAX". As
>      > mentioned in the original email, the RKMAX was calculated from cutoff
>      > in EV using the scheme from userguide (sqrt(cutoff in Ryd) * size of
>      > smallest sphere). Specific values differed between cells, smallest
>      > spheres were around 1.5 and the RKMAX was around 8.0 (PAW cutoff of
>      > 400eV).
>      >>
>      >> 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
>      >> ?
>      >
>      > default
>      >
>      >> spin-polarization ??
>      >
>      > no
>      >
>      >> Is the PAW done with VASP and a fairly new release (accurate PAW
>      >> potentials ?)
>      >
>      > It was VASP and it was one of the latest versions. Will check for
>      > specifics.
>      >
>      >>
>      >> 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
>      >>> _______________________________________________
>      >>> Wien mailing list
>      >>> Wien at 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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>
>     --
>
>     P.Blaha
>     --------------------------------------------------------------------------
>     Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
>     Phone: +43-1-58801-165300 FAX: +43-1-58801-165982
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-- 

                                       P.Blaha
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