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> 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>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> 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> 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> 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 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 >>>> 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 >> >> _______________________________________________ >> Wien mailing list >> 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20130301/62c78442/attachment.htm>
