[Wien] When to add a U.

[L. D. Marks]

Of course Mg does not have d-electrons, perhaps a bad choice.

The main question is when to add a U:
	a) Always - with some guess/estimate?
	b) Only after estimating it from a supercell calculation (not so
simple and not always possible)?
	c) Only when you can't explain experimental data, with the value
of U chosen to "agree with experiment" - in which case it is really a fudge.

On Mon, 24 Jan 2005, Peter Blaha wrote:

> Ca is know to have "almost" active 3d electrons (the next element in the
> periodic table, Sc,  of course has).
> E.g. in an LCAO description, an accurate calculation would need d-basis
> fuctions for Ca. Thus it
> is reasonable and clear that LDA+U for Ca-d has an effect (I'll not duscuss
> whether it is physically meaningful or not, but it might be that the Ca-d
> bands are not fully correctly positioned within LDA alone !).
>
> Mg does NOT have d-electrons! An LCAO calculation does not need d-basis
> fuctions. So putting LDA+U on Mg-d is meaningless!
> The  d-partial waves of Mg within our APW formalism are tails of O-p
> wavefunctions!
> If you want to play around, you may want to add LDA+U to O-p ! (or
> Mg s,p - although I'd call the latter also "unphysical").
>
>
> > 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....??
> > > 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.
>
>
>                                       P.Blaha
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
> Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
> --------------------------------------------------------------------------
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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
-----------------------------------------------