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

[Kamil Klier]

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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>> > > >Wien at zeus.theochem.tuwien.ac.at
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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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