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.<br><br><div class="gmail_quote">
On Fri, Mar 1, 2013 at 1:24 PM, Zsolt Rak <span dir="ltr"><<a href="mailto:zsolt.rak@gmail.com" target="_blank">zsolt.rak@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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I want to calculate the most accurate bulk energy and the a-priori information I have are the atomic positions.<br>
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<div class="gmail_quote">On Fri, Mar 1, 2013 at 1:59 PM, Laurence Marks <span dir="ltr">
<<a href="mailto:L-marks@northwestern.edu" target="_blank">L-marks@northwestern.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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?
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<div>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.
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<div class="gmail_quote">On Fri, Mar 1, 2013 at 12:47 PM, Zsolt Rak <span dir="ltr">
<<a href="mailto:zsolt.rak@gmail.com" target="_blank">zsolt.rak@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>Dear wien2k users, <br>
<br>
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.
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I would appreciate further thoughts and insights into this issue.<br>
<br>
Thank you, <br>
Zs </div>
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<span><font color="#888888">-- <br>
Professor Laurence Marks<br>
Department of Materials Science and Engineering<br>
Northwestern University<br>
<a href="http://www.numis.northwestern.edu" target="_blank">www.numis.northwestern.edu</a>
<a href="tel:1-847-491-3996" value="+18474913996" target="_blank">1-847-491-3996</a><br>
"Research is to see what everybody else has seen, and to think what nobody else has thought"<br>
Albert Szent-Gyorgi </font></span></div>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br>Professor Laurence Marks<br>Department of Materials Science and Engineering<br>Northwestern University<br><a href="http://www.numis.northwestern.edu" target="_blank">www.numis.northwestern.edu</a> 1-847-491-3996<br>
"Research is to see what everybody else has seen, and to think what nobody else has thought"<br>Albert Szent-Gyorgi