[Wien] volume optimization

[Laurence Marks]

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
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