[Wien] Okay to combine mBJ, spinorbit, and LDA+U?

Sat Aug 18 18:51:30 CEST 2012

I think this discussion is good, but I want to introduce some caveats. Dft
is a ground state model, so it does not have to get excited state
properties right. For instance, there is no density in the conduction band
for ZnO so the exchange correllation terms for the conduction band are not
considered.

If adding U is better for a ground state property then it is fine; adding
it just to get a better apparent gap concerns me. While it is certainly
true that, for instance, exact exchange is better for gaps, it is also
better for some ground state properties so is reasonable. The BJ model has
other justification as an effective potential as Peter mentioned earlier,
so is not simply an empirical fudge.

---------------------------
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu 1-847-491-3996
"Research is to see what everybody else has seen, and to think what nobody
else has thought"
Albert Szent-Gyorgi
On Aug 18, 2012 11:27 AM, "Kamil Klier" <kk04 at lehigh.edu> wrote:

> Thanks Fabien,
>
> Indeed the match between MBJLDA theory and experiment (Fig. 1 in the
> quoted paper) is excellent.
>
> However, the bandgap of ZnO is underpredicted by mBJ only, reading
> from Fig. 1 ca. 2.8 eV.  This would render zinc oxide colored in the
> visible region, but pure ZnO is white, in accord with experimental
> bandgap 3.2 - 3.4 eV.  Moreover, the Zn3d_10 (filled) narrow band
> falls below the O2p valence band (from VB XPS) while a quick
> calculation with MBJLDA results in blending of O2p and Zn3d.  Is it
> possible that in the ZnO case the U (say U_eff = 0.46 Ry) would help a
> bit as follows: it would push the Zn3d down and improve the bandgap -
> unless of course there are theoretical reasons why mBJ and U should be
> in conflict.  That does not seem to be the case, however, for 4f
> orbitals.
>
> Best regards,
>
> Kamil Klier
>
> Quoting tran at theochem.tuwien.ac.at:
>
> > Yes, mBJ alone is already ok for NiO:
> > http://prl.aps.org/abstract/PRL/v102/i22/e226401
> > So, adding U is not a good idea.
> >
> > On Sat, 18 Aug 2012, Kamil Klier wrote:
> >
> >> The Wien example for NiO uses U_eff = 0.52 Ry for the Ni3d orbitals.
> >>
> >> Would that mean that using subsequent mBJ potential for 'improvement of
> >> bandgap of NiO' is not appropriate or at least is an overkill?
> >>
> >> Best regards,
> >>
> >> Kamil Klier
> >>
> >> Quoting Peter Blaha <pblaha at theochem.tuwien.ac.at>:
> >>
> >> >mBJ+U is appropriate for 4f systems only (because mBJ is too weak to
> fully
> >> >localize the 4f electrons). Do not use it for d-electrons.
> >> >
> >> >mBJ is made to give a good bandstructure. We have evidence, that the
> >> >resulting
> >> >electron density is too ionic, thus a force optimization using MSR1a
> could
> >> >be problematic (although it could be better than GGA in some cases
> (with 3d
> >> >electrons - Jahn-Teller distortions).
> >> >Eventually, MSR1a with the original BJ potential (c=1) is physically
> more
> >> >justified, sind original BJ is an approximation to OEP (optimized
> effective
> >> >potential),
> >> >which should be close to the "exact local exchange-only" potential.
> >> >(Note that an "exact exchange potential" + LDA-correlation can be much
> more
> >> >wrong than plain LDA !!!!! for certain cases, because we miss the error
> >> >cancellation)
> >> >
> >> >before doing Spin-orbit calculations, I'd remove case.in0_ggr and use
> the
> >> >case.grr file from the scf-mBJ calculation without SO. I do not trust
> >> >the kinetic energy densities with SO.
> >> >
> >> >
> >> >Am 16.08.2012 22:15, schrieb Laurence Marks:
> >> > >If it is a decent insulator I would do LDA+U directly; often it
> >> > >converges better and since the lattice parameter and forces change
> you
> >> > >do not gain much by first doing LDA/GGA. Normally LDA+U is stable,
> >> > >often more stable that LDA/GGA. Volume optimization should be done
> >> > >first, then min_lapw or MSR1a. Better is to do MSR1a or min_lapw at
> >> > >each volume.
> >> > >
> >> > >Then add -so, mBJ as appropriate with the optimized positions.
> >> > >However, I not sure if mBJ+U is appropriate (I doubt that it is). It
> >> > >might be that LDA+U positions are a better approximation for mBJ, not
> >> > >sure. One way is to minimize the forces with mBJ using MSR1a (not
> >> > >min_lapw/PORT) and compare them to LDA+U. If they are the same then
> >> > >you are in good shape, needs testing. Maybe someone has....
> >> > >
> >> > >N.B., it is completely fine to minimize positions in mBJ using MRS1a
> >> > >-- do not use min_lapw/PORT, it will not be correct. MSR1a does not
> >> > >care that the energy is incorrect whereas min_lapw/PORT does.
> >> > >
> >> > >On Thu, Aug 16, 2012 at 2:44 PM, Jeff Spirko <spirko at lehigh.edu>
> wrote:
> >> > > >Is it okay to use spinorbit and LDA+U with mBJ?
> >> > > >
> >> > > >I would guess it is done like this:
> >> > > > * Check that forces <10 mRy/au with plain LDA or GGA.  Reduce via
> >> > > > min_lapw.
> >> > > > * Volume optimization (if desired) with plain LDA or GGA to reduce
> >> > > >absolute pressure.
> >> > > > * Set up LDA+U (Sec 4.5.6) and use -orb flag from now on.
> >> > > > * Need to converge LDA+U???
> >> > > > * Follow mBJ instructions (Sec 4.5.9).
> >> > > > * After mBJ+LDA+U is converged, follow spinorbit instructions (Sec
> >> > > > 4.5.5).
> >> > > > * For spinpolarized, check whether atoms became nonequivalent
> >> > > >(affects case.inso, case.inorb,
> >> > > >    case.indmc, case.in1c, basically any input file with atom
> lists or
> >> > > > indices)
> >> > > > * touch .fulldiag (necessary because klist can change???)
> >> > > > * Do final run with -so -orb
> >> > > >
> >> > > >Best regards,
> >> > > >--
> >> > > >Jeff Spirko   spirko at lehigh.edu   WD3V   |=>
> >> > > >
> >> > > >The study of non-linear physics is like the study of non-elephant
> >> > > >biology.
> >> > > >_______________________________________________
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> >> > >
> >> > >
> >> > >
> >> >
> >> >--
> >> >-----------------------------------------
> >> >Peter Blaha
> >> >Inst. Materials Chemistry, TU Vienna
> >> >Getreidemarkt 9, A-1060 Vienna, Austria
> >> >Tel: +43-1-5880115671
> >> >Fax: +43-1-5880115698
> >> >email: pblaha at theochem.tuwien.ac.at
> >> >-----------------------------------------
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> >>
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