[Wien] When to add a U.

[L. D. Marks]

I'm the guilty party for the question, which I agree with Georg is
important. For instance, there is a paper by Fang and Terakura, Surf.Sci.
470, L75 (2000) where LDA+U is used for a SrTiO3 surface (a topic dear to
my heart) where there are large effects and they argue that LDA+U is in
better agreement with experiment.

Just to expand the scope of the discussion (?) further, for fun I did
LDA+U for MgO. With U=6eV, J=0 there is a change of about 0.01 Ryd/Mg
atom. Although MgO has no "d-electrons", it does have d-harmonics in the
density and if I understand DFT/U correctly, there is no distinction
between d-electrons and d-harmonics. This is probably similar to the
effects in Ca....??

On Sun, 23 Jan 2005 georg at chem.au.dk wrote:

> After posting the paper
> > > http://xxx.lanl.gov/pdf/cond-mat/0412560
> about calculating the U, a question was posted:
> > Your paper almost implies (or does imply?) that one should always use a U
> > for a transition metal (oxide?) -- or am I reading this wrong?
>
> I'd like to answer it in this public forum, because I'd like to start a
> discussion. My understanding of the subject isn't perfect and I'd be happy for
> any comments
>
> My opinion is yes and no.
>
> YES because,
> The LDA and GGA are only exact for the free electron gas and for all other
> systems the spurious self interaction is a source off error. Off course both
> the effect and the size off the U is the largest for the "strongly correlated
> systems". However I recently read a nice paper where they calculated NMR
> shielding parameters, which depend on the unoccupied bands:
> J. Am. Chem. Soc.; 2004; 126(39); 12628-12635
> The conclusion here is that one need a U for the unoccupied d-states of Ca.
> This
> would off course not influence any of the ground state properties, but it
> surprised me because I would guess the shape off the d-state to be highly
> diffuse.
>
>
> NO because:
> LDA and GGA are functionals with well defined potentials and energyfunctional.
> The LDA+U is a basis-set dependent method with a poorly defined
> energy-functional. One should concentrate on doing the LDA and GGA well (not
> introducing unphysical atomic sphere approximations andthe like) and understand
> when and why it might fail. David Singh stated this oppion. In a recent example
> one would have overlooked a structural distortion, if the a disagreement with
> experiment had just been blamed on some magical U.
> P. Blaha, D.J. Singh, K. Schwarz, Physical Review Letters 93, 216403 (2004)
> Another example might be that you force a good agreement with experiment when
> you need to describe many-body effects that are also not described with LDA+U
> Phys Rev B , 67, 153106 (2003)
>
> These are my two Eurocents contribution.
>
> >
> > On Tue, 11 Jan 2005 georg at chem.au.dk wrote:
> >
> > > Hi all
> > >
> > > there have been some request about calculating U in APW. I have uploaded a
> > paper
> > > where we do this for magnetite. Furthermore it is done for NiO and and FeO
> > and
> > > Fe2O3 as tests:
> > > it should appear in EPL soon.
> > >
> > > The method should be described in there. I'll try and send a worked out
> > example
> > > for NiO when I get time.
> > >
> > >  Best wishes
> > >    Georg
>
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-----------------------------------------------
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60201, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L - marks @ northwestern . edu
http://www.numis.northwestern.edu
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