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

[tran at theochem.tuwien.ac.at]

Lattice relaxation and atomic positions relaxation are two different
things. In WIEN2k there is no automatic lattice relaxation
(the stress tensor would need to be implemented), i.e., the
lattice constants a, b and c can only be optimized manually (with
x optimize).

The automatic (using forces) relaxation of atomic positions
is possible only for functionals for which the forces are calculated
correctly (GGA, GGA+U, onsite hybrids). This is not the case with
spin-orbit coupling or the full hybrid functionals recently implemented.

For mBJ, the problem is a little bit different. The forces are calculated
correctly, but (in my opinion) are nonsense and should not be used
for the atomic position relaxation. The problem is that mBJ is a potential
for which there is no associated energy functional, and therefore the
forces will be zero for a geometry which does not correspond to a
minimum on a total-energy surface which does not even exist.

My recommendation is to never use mBJ for structure optimization
(lattice constant and atomic positions).

F. Tran

On Fri, 17 Aug 2012, Zhiyong Zhu wrote:

> Dear Prof. Laurence Marks,
> 
> I have a question about the force minimization in mBJ calculations.
> According to the Userguide, the forces in mBJ calculations are not
> correct, which means that the lattice relaxation using forces on atoms
> in mBJ calculations is impossible. According to your description below,
> however, it seems that the lattice relaxation is possible even in mBJ
> calculations, if we use MSR1a in *.inm file. Is my understanding correct?
> 
> Another question is about the force minimization in calculations with
> spin-orbit coupling. According to the wien2k website
> (http://www.wien2k.at/reg_user/limitations/), lattice relaxation in calculations
> with spin-orbit coupling is also not possible, because the Pulay corrections
> to the forces are not correct in the presence of spin-orbit coupling.
> So my concern is that whether the MSR1a option in *.inm file can overcome
> this problem?
> 
> Thank you very much.
> 
> Regards,
> Zhiyong Zhu
> 
> On 8/16/12, Laurence Marks <L-marks at northwestern.edu> wrote:
> > 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
> >> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> >
> >
> >
> > --
> > 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
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> >
> 
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