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<div class="moz-cite-prefix">Dear Iurii, dear Giuseppe,</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">thank you for the interesting
discussion. My "knowledge" comes from papers using empirical U
values. I see that my question is still related to an ongoing
debate. However, your arguments and references helped my for my
discussion. Thanks a lot,</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">best regards,</div>
<div class="moz-cite-prefix">Malte<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Am 29.10.20 um 18:36 schrieb Timrov
Iurii:<br>
</div>
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<p>Dear Giuseppe,</p>
<p><br>
</p>
<p>Thank you for your comment!</p>
<p><br>
</p>
<p><font size="2"><span style="font-size:10pt">> In
principle you are right, but if you try to compute,
e.g., formation
<br>
> heats or formation energy of defects by using
different U values (I <br>
> did it long time ago in the case of TiO2 related
studies for metallic <br>
> Ti, for TiO2, for Ti2O3) you fall sometimes very
far from experimental <br>
> references, farther then using non corrected GGA.</span></font></p>
<p><font size="2"><span style="font-size:10pt"><br>
</span></font></p>
<p><font size="2"><span style="font-size:10pt"><span
style="font-size:12pt;
font-family:Arial,Helvetica,sans-serif">Ok, I see. So</span><span
style="font-size:12pt;
font-family:Arial,Helvetica,sans-serif"> you used
semiempirical U and things got worse, right? It would
be interesting to see what happens if ab initio U is
used for each structure.</span><br>
</span></font></p>
<p><font size="2"><span style="font-size:10pt"><br>
</span></font></p>
<p><font size="2"><span style="font-size:10pt">> I've used
semiempirically averaged values in a lot of studies
(including the
<br>
> correction of O- and N-related bands in oxides and
nitrides) and <br>
> things are a bit semiempirical, obviously, but
still quite improved <br>
> with respect to GGA. As long as you motivate what
you are doing, I do <br>
> not see particular harm in semiempirical U
corrections!</span></font><br>
</p>
<p><br>
</p>
<p>Ok. Of course, one is free to choose whether to use a
semiempirical or ab initio U. But I see the following issues
when using semiempirical U:</p>
<p>- choice of the Hubbard manifold. U depends a lot on the
Hubbard manifold (atomic, ortho-atomic, Wannier functions,
etc.). Different codes use different Hubbard manifolds. So
if one takes U from some paper where it was determined
semiempirically (e.g. using VASP) and use it in QE (with a
different manifold) the results can be totally different</p>
<p>- choice of the pseudopotential (see Appendix in J. Chem.
Phys. 129, 134314 (2008)). From this reference you can see
that the pseudopotentials generated in different oxidation
states require different U. It is easy to compute U for a
given pseudo, but how to determine it semiempirically (try
to reproduce some experimental quantity? and what to do if
there is no experimental data for the material under
investigation?)</p>
<p>- choice the exchange-correlation functional: PW91, PBE,
PBEsol, ... - each time the U correction is not exactly the
same. For each functional U must be tuned by hand if done
semiempirically.
</p>
<p>So for a given computational setup (Hubbard manifold,
pseudos, functional, etc.) I would simply compute U from
first principles (luckily we have now theories and codes to
do that, which was not the case in the past). And when one
wants to do a high-throughput study for thousands of
materials, fitting U for each system to the experimental
data would kill the whole idea of high-throughput. Just my
thoughts.
<br>
</p>
<p><br>
</p>
<p>Thanks!<br>
</p>
<p><br>
</p>
<p>Greetings,</p>
<p>Iurii<br>
</p>
<p><br>
</p>
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<font size="3" face="'Times New Roman', Times, serif"
color="808080">--<br>
Dr. Iurii TIMROV<br>
Postdoctoral Researcher<br>
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<div id="x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
face="Calibri, sans-serif" color="#000000"><b>From:</b>
users <a class="moz-txt-link-rfc2396E" href="mailto:users-bounces@lists.quantum-espresso.org"><users-bounces@lists.quantum-espresso.org></a> on
behalf of Giuseppe Mattioli
<a class="moz-txt-link-rfc2396E" href="mailto:giuseppe.mattioli@ism.cnr.it"><giuseppe.mattioli@ism.cnr.it></a><br>
<b>Sent:</b> Thursday, October 29, 2020 5:58:39 PM<br>
<b>To:</b> Quantum ESPRESSO users Forum<br>
<b>Subject:</b> Re: [QE-users] Thermodynamics with DFT+U</font>
<div> </div>
</div>
</div>
<font size="2"><span style="font-size:10pt;">
<div class="PlainText"><br>
Dear Iurii<br>
In principle you are right, but if you try to compute, e.g.,
formation <br>
heats or formation energy of defects by using different U
values (I <br>
did it long time ago in the case of TiO2 related studies for
metallic <br>
Ti, for TiO2, for Ti2O3) you fall sometimes very far from
experimental <br>
references, farther then using non corrected GGA. I've used
<br>
semiempirically averaged values in a lot of studies
(including the <br>
correction of O- and N-related bands in oxides and nitrides)
and <br>
things are a bit semiempirical, obviously, but still quite
improved <br>
with respect to GGA. As long as you motivate what you are
doing, I do <br>
not see particular harm in semiempirical U corrections!<br>
Best<br>
Giuseppe<br>
<br>
Quoting Timrov Iurii <a class="moz-txt-link-rfc2396E" href="mailto:iurii.timrov@epfl.ch"><iurii.timrov@epfl.ch></a>:<br>
<br>
> Dear Malte,<br>
><br>
><br>
> This is not an easy question. Let me express my
opinion.<br>
><br>
><br>
>> I read many times in papers that I should not
compare total energies of<br>
>> DFT+U calculations with different U values.<br>
><br>
><br>
> Can you give some references please?<br>
><br>
><br>
> Well the problem is that in literature often people
still use <br>
> empirical values of U. In this case, I agree that the
comparison of <br>
> energies will depend strongly on U which one chooses
based on some <br>
> arguments. But if one computes U from first principles,
then U is a <br>
> response property of each system (and the response is
different in <br>
> each system, hence different U) then it makes sense to
me to compare <br>
> energies (but I am not aware of any theoretical proof
that it is <br>
> allowed to do so).<br>
><br>
><br>
> For example, in LiCoO2 the computed value of U for
Co-3d is 6.91 eV, <br>
> while U for Co-3d in CoO is 4.55 eV (both using
'atomic' orbitals <br>
> and some pseudos on top of PBEsol - see PRB 101, 064305
(2020)). So <br>
> in both cases we consider Co-3d states, but the value
of U is very <br>
> different. This is so because Co-3d states require
different <br>
> corrections when they are in different chemical
environments (these <br>
> Co-3d states hybridize differently with ligands in
different <br>
> compounds). If you use an average value of 5.73 eV for
both systems, <br>
> then this value is not a response property of any of
these systems <br>
> (I do not know what it is in this case).<br>
><br>
><br>
> One can also see DFT+U as DFT with a Hubbard
functional: the U <br>
> correction is different for different systems (why
should it be the <br>
> same?). It's like hybrids: people tune the value of
alpha in PBE0 <br>
> for each system (alpha is related to the inverse of the
dielectric <br>
> constant), and alpha is different for different systems
(because the <br>
> dielectric constant is different). So here is the same,
U is <br>
> different for different systems: different systems need
different <br>
> amount of the corrections to restore the piece-wise
linearity of the <br>
> energy (PRB 71, 035105 (2005)) and alleviate
self-interactions.<br>
><br>
><br>
> The comparison of energies with different ab initio U
values was <br>
> done in these papers: PRB 99, 094102 (2019); PRR 2,
023313 (2020); <br>
> PRM 3, 033801 (2019).<br>
><br>
><br>
>> What should I do in case of complex hull
calculations of a phase diagram?<br>
><br>
><br>
> I would compute U for each phase and compare energies.<br>
><br>
><br>
>> With QE it is now possible to calculate the U
values for each <br>
>> composition by the hp.x code. Can I<br>
> compare the resulting total energies with each other
safely?<br>
><br>
><br>
> Well there is no general consensus on this issue, in
part because <br>
> still many people use empirical U. I think that if U is
computed ab <br>
> initio (i.e. it is really a response property of each
material) then <br>
> it makes sense to compare energies with different U
values (U <br>
> computed for each system). But people are still
investigating this <br>
> issue.<br>
><br>
><br>
> I will be happy to hear other opinions, and to hear <br>
> corrections/remarks if something what I said is
misleading.<br>
><br>
><br>
> Greetings,<br>
><br>
> Iurii<br>
><br>
><br>
> --<br>
> Dr. Iurii TIMROV<br>
> Postdoctoral Researcher<br>
> STI - IMX - THEOS and NCCR - MARVEL<br>
> Swiss Federal Institute of Technology Lausanne (EPFL)<br>
> CH-1015 Lausanne, Switzerland<br>
> +41 21 69 34 881<br>
> <a href="http://people.epfl.ch/265334"
moz-do-not-send="true">http://people.epfl.ch/265334</a><br>
> ________________________________<br>
> From: users
<a class="moz-txt-link-rfc2396E" href="mailto:users-bounces@lists.quantum-espresso.org"><users-bounces@lists.quantum-espresso.org></a> on behalf
of <br>
> Giuseppe Mattioli <a class="moz-txt-link-rfc2396E" href="mailto:giuseppe.mattioli@ism.cnr.it"><giuseppe.mattioli@ism.cnr.it></a><br>
> Sent: Thursday, October 29, 2020 3:52:48 PM<br>
> To: Quantum ESPRESSO users Forum<br>
> Subject: Re: [QE-users] Thermodynamics with DFT+U<br>
><br>
><br>
> Dear Malte<br>
> I suppose that, as a trivial rule of thumb, if U values
are not so<br>
> different then you can use some average value for all
systems.<br>
> HTH<br>
> Giuseppe<br>
><br>
> Quoting Malte Sachs
<a class="moz-txt-link-rfc2396E" href="mailto:malte.sachs@chemie.uni-marburg.de"><malte.sachs@chemie.uni-marburg.de></a>:<br>
><br>
>> Dear all,<br>
>><br>
>> I read many times in papers that I should not
compare total energies<br>
>> of DFT+U calculations with different U values. What
should I do in<br>
>> case of complex hull calculations of a phase
diagram? With QE it is<br>
>> now possible to calculate the U values for each
composition by the<br>
>> hp.x code. Can I compare the resulting total
energies with each<br>
>> other safely?<br>
>><br>
>> Best regards,<br>
>> Malte<br>
>><br>
>> --<br>
>> Malte Sachs<br>
>> Anorganische Chemie, Fluorchemie<br>
>> Philipps-Universität Marburg<br>
>> Hans-Meerwein-Straße 4<br>
>> 35032 Marburg (Paketpost: 35043 Marburg)<br>
>> Tel.: +49 (0)6421 28 - 25 68 0<br>
>> <a href="http://www.uni-marburg.de/fb15/ag-kraus/"
moz-do-not-send="true">http://www.uni-marburg.de/fb15/ag-kraus/</a><br>
><br>
><br>
><br>
> GIUSEPPE MATTIOLI<br>
> CNR - ISTITUTO DI STRUTTURA DELLA MATERIA<br>
> Via Salaria Km 29,300 - C.P. 10<br>
> I-00015 - Monterotondo Scalo (RM)<br>
> Mob (*preferred*) +39 373 7305625<br>
> Tel + 39 06 90672342 - Fax +39 06 90672316<br>
> E-mail: <a class="moz-txt-link-rfc2396E" href="mailto:giuseppe.mattioli@ism.cnr.it"><giuseppe.mattioli@ism.cnr.it></a><br>
><br>
> _______________________________________________<br>
> Quantum ESPRESSO is supported by MaX <br>
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<br>
<br>
<br>
GIUSEPPE MATTIOLI<br>
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA<br>
Via Salaria Km 29,300 - C.P. 10<br>
I-00015 - Monterotondo Scalo (RM)<br>
Mob (*preferred*) +39 373 7305625<br>
Tel + 39 06 90672342 - Fax +39 06 90672316<br>
E-mail: <a class="moz-txt-link-rfc2396E" href="mailto:giuseppe.mattioli@ism.cnr.it"><giuseppe.mattioli@ism.cnr.it></a><br>
<br>
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<pre class="moz-signature" cols="72">--
Malte Sachs
Anorganische Chemie, Fluorchemie
Philipps-Universität Marburg
Hans-Meerwein-Straße 4
35032 Marburg (Paketpost: 35043 Marburg)
Tel.: +49 (0)6421 28 - 25 68 0
<a class="moz-txt-link-freetext" href="http://www.uni-marburg.de/fb15/ag-kraus/">http://www.uni-marburg.de/fb15/ag-kraus/</a></pre>
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