[Wien] volume optimization
Rocquefelte
xavier.rocquefelte at cnrs-imn.fr
Sat Mar 2 10:28:24 CET 2013
The best theoretical approach is the one mentioned by Laurence in his
last message.
About your last question ... It is clearly a matter of taste because
whatever we do, an approach consisting to choose the Ueff value by
ajusting it for one property is "semi-empirical". I never did such an
asjustment of Ueff ... just based on the cell volume or structural
properties and then use the Ueff for electronic properties.
However, it could make sense, because the structural properties are the
consequence of the electronic structure (orbital interactions) and thus
a good description of the structural properties should imply a good
description of the electronic structure ... However, you can also
artificially stabilize the experimental volume for bad reasons!!! It is
why I prefer to have one more property (optical, magnetic, NMR, ...) to
compare with my calculation in order to validate the Ueff value.
I should mentioned that depending on what you are doing the Ueff value
could be different! Indeed fixing the Ueff based on magnetic properties
or optical properties will not lead to the same Ueff value ... This is
mainly related to the fact that when we adjust the Ueff value based on
an experimental value we correct not only the deficiency of DFT to
properly describe the localized d- or f-states, but more ... In that
sense adjusting the Ueff value on an experimental value is not correct
but it remains a pragmatic approach (with clear limitations).
In the other hand, the Ueff value deduded from the constrained approach
will lead to results that will not perfectly reproduce the experiments,
but these results will correspond to the best you can do using DFT with
a "correct" Ueff correction. It is why I consider that it remains a
matter of taste and why I feel that you need other experimental data to
insure that your Ueff parameter is "relevant in a certain context".
Best Regards
Xavier
Le 3/1/2013 9:12 PM, Zsolt Rak a écrit :
> Dear Xavier, you said that you choose the Ueff value to reproduce one
> property and then you use this Ueff value to predict the others. My
> question is can I choose Ueff to reproduce a structural property (such
> as the lattice constant or cell volume) and than use that value of
> Ueff to predict electronic properties?
>
>
>
> On Fri, Mar 1, 2013 at 3:04 PM, Laurence Marks
> <L-marks at northwestern.edu <mailto:L-marks at northwestern.edu>> wrote:
>
> If all you have are the atomic positions (and lattice constant)
> then the only unbiased approach would be to calculate the U, see
> http://www.wien2k.at/reg_user/textbooks/Constraint_U.pdf.
>
> N.B., the DFT+U method is variational for a fixed U, but I don't
> think it is variational as a function(al) of U.
>
> On Fri, Mar 1, 2013 at 1:58 PM, Zsolt Rak <zsolt.rak at gmail.com
> <mailto:zsolt.rak at gmail.com>> wrote:
>
> Let's suppose that the atomic positions (and lattice
> constants) are the only information that I have. Is it
> physically justified to use LDA+U/GGA+U to optimize the
> volume? or to tune the U value to reproduce the experimental
> lattice constants? Also, is the DFT+U method based on the
> variational principle?
>
> On Fri, Mar 1, 2013 at 2:43 PM, Laurence Marks
> <L-marks at northwestern.edu <mailto:L-marks at northwestern.edu>>
> wrote:
>
> You need more a-priori information than this, for instance
> the bulk energy of related compounds for which a U is
> relevant. Tuning the U to reproduce known data is not by
> itself spectacular science.
>
> On Fri, Mar 1, 2013 at 1:24 PM, Zsolt Rak
> <zsolt.rak at gmail.com <mailto:zsolt.rak at gmail.com>> wrote:
>
> I want to calculate the most accurate bulk energy and
> the a-priori information I have are the atomic positions.
>
>
>
> On Fri, Mar 1, 2013 at 1:59 PM, Laurence Marks
> <L-marks at northwestern.edu
> <mailto:L-marks at northwestern.edu>> wrote:
>
> My two cents. Both LDA+U and GGA+U are wrong. That
> said, for f-/d- systems they are often better than
> LDA/GGA for some properties. The question you
> should ask yourself is what property are you
> trying to measure/predict, and what a-priori
> information (reference state) do you have that can
> be used?
>
> For instance, if I want to calculate a surface
> energy then I would tune the U to give the most
> accurate bulk energy treating this as my a-priori
> information; similarly if I wanted to calculate
> the elastic behavior of a defect I would tune to
> the bulk elastic constants. In my opinion this is
> the only justifiable approach.
>
>
> On Fri, Mar 1, 2013 at 12:47 PM, Zsolt Rak
> <zsolt.rak at gmail.com <mailto:zsolt.rak at gmail.com>>
> wrote:
>
> Dear wien2k users,
>
> I am calculating the properties of several
> f-electron compounds. I would like to ask the
> users' opinion about the volume optimization
> in an f- or d-electron system: which way is
> better (or physically justified), with LDA/GGA
> or with LDA+U/GGA+U? In my opinion, the
> LDA+U/GGA+U techniques were developed to
> correct band energies of localized states, so
> there is no fundamental physical reason to use
> LDA+U/GGA+U methods for volume optimization.
> However, we observe a change in the lattice
> parameters when we go from LDA/GGA to
> LDA+U/GGA+U. Also, from a brief search of the
> literature we found that, in many cases,
> people tune the Hubbard-U parameter to
> reproduce the experimental lattice constants.
> I would appreciate further thoughts and
> insights into this issue.
>
> Thank you,
> Zs
>
>
>
>
> --
> Professor Laurence Marks
> Department of Materials Science and Engineering
> Northwestern University
> www.numis.northwestern.edu
> <http://www.numis.northwestern.edu> 1-847-491-3996
> <tel: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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>
>
>
>
> --
> Professor Laurence Marks
> Department of Materials Science and Engineering
> Northwestern University
> www.numis.northwestern.edu
> <http://www.numis.northwestern.edu> 1-847-491-3996
> <tel:1-847-491-3996>
> "Research is to see what everybody else has seen, and to
> think what nobody else has thought"
> Albert Szent-Gyorgi
>
> _______________________________________________
> Wien mailing list
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>
>
>
>
> --
> Professor Laurence Marks
> Department of Materials Science and Engineering
> Northwestern University
> www.numis.northwestern.edu <http://www.numis.northwestern.edu>
> 1-847-491-3996 <tel: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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