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

[Peter Blaha]

Yes, the Zn-3d states are very localized (like the 4f) and some U (much smaller than what you would use in GGA+U)
will give you the experimental gap.

Alternatively you can use modified mBJ parameters (PRB 85, 155109) which are tailored more towards semiconductors
(gaps below 5 eV) and bring the ZnO gap above 3.2 eV.

Am 18.08.2012 18:27, schrieb 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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>>> > >
>>> > >
>>> > >
>>> >
>>> >--
>>> >-----------------------------------------
>>> >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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-- 
-----------------------------------------
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
-----------------------------------------