[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
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> >>> 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
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--
P.Blaha
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