Dear Iurii
In principle you are right, but if you try to compute, e.g., formation
heats or formation energy of defects by using different U values (I
did it long time ago in the case of TiO2 related studies for metallic
Ti, for TiO2, for Ti2O3) you fall sometimes very far from experimental
references, farther then using non corrected GGA. I've used
semiempirically averaged values in a lot of studies (including the
correction of O- and N-related bands in oxides and nitrides) and
things are a bit semiempirical, obviously, but still quite improved
with respect to GGA. As long as you motivate what you are doing, I do
not see particular harm in semiempirical U corrections!
Best
Giuseppe
Quoting Timrov Iurii <[email protected]>:
Dear Malte,
This is not an easy question. Let me express my opinion.
I read many times in papers that I should not compare total energies of
DFT+U calculations with different U values.
Can you give some references please?
Well the problem is that in literature often people still use
empirical values of U. In this case, I agree that the comparison of
energies will depend strongly on U which one chooses based on some
arguments. But if one computes U from first principles, then U is a
response property of each system (and the response is different in
each system, hence different U) then it makes sense to me to compare
energies (but I am not aware of any theoretical proof that it is
allowed to do so).
For example, in LiCoO2 the computed value of U for Co-3d is 6.91 eV,
while U for Co-3d in CoO is 4.55 eV (both using 'atomic' orbitals
and some pseudos on top of PBEsol - see PRB 101, 064305 (2020)). So
in both cases we consider Co-3d states, but the value of U is very
different. This is so because Co-3d states require different
corrections when they are in different chemical environments (these
Co-3d states hybridize differently with ligands in different
compounds). If you use an average value of 5.73 eV for both systems,
then this value is not a response property of any of these systems
(I do not know what it is in this case).
One can also see DFT+U as DFT with a Hubbard functional: the U
correction is different for different systems (why should it be the
same?). It's like hybrids: people tune the value of alpha in PBE0
for each system (alpha is related to the inverse of the dielectric
constant), and alpha is different for different systems (because the
dielectric constant is different). So here is the same, U is
different for different systems: different systems need different
amount of the corrections to restore the piece-wise linearity of the
energy (PRB 71, 035105 (2005)) and alleviate self-interactions.
The comparison of energies with different ab initio U values was
done in these papers: PRB 99, 094102 (2019); PRR 2, 023313 (2020);
PRM 3, 033801 (2019).
What should I do in case of complex hull calculations of a phase diagram?
I would compute U for each phase and compare energies.
With QE it is now possible to calculate the U values for each
composition by the hp.x code. Can I
compare the resulting total energies with each other safely?
Well there is no general consensus on this issue, in part because
still many people use empirical U. I think that if U is computed ab
initio (i.e. it is really a response property of each material) then
it makes sense to compare energies with different U values (U
computed for each system). But people are still investigating this
issue.
I will be happy to hear other opinions, and to hear
corrections/remarks if something what I said is misleading.
Greetings,
Iurii
--
Dr. Iurii TIMROV
Postdoctoral Researcher
STI - IMX - THEOS and NCCR - MARVEL
Swiss Federal Institute of Technology Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
+41 21 69 34 881
http://people.epfl.ch/265334
________________________________
From: users <[email protected]> on behalf of
Giuseppe Mattioli <[email protected]>
Sent: Thursday, October 29, 2020 3:52:48 PM
To: Quantum ESPRESSO users Forum
Subject: Re: [QE-users] Thermodynamics with DFT+U
Dear Malte
I suppose that, as a trivial rule of thumb, if U values are not so
different then you can use some average value for all systems.
HTH
Giuseppe
Quoting Malte Sachs <[email protected]>:
Dear all,
I read many times in papers that I should not compare total energies
of DFT+U calculations with different U values. What should I do in
case of complex hull calculations of a phase diagram? With QE it is
now possible to calculate the U values for each composition by the
hp.x code. Can I compare the resulting total energies with each
other safely?
Best regards,
Malte
--
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
http://www.uni-marburg.de/fb15/ag-kraus/
GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <[email protected]>
_______________________________________________
Quantum ESPRESSO is supported by MaX
(www.max-centre.eu<http://www.max-centre.eu>)
users mailing list [email protected]
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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <[email protected]>
_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list [email protected]
https://lists.quantum-espresso.org/mailman/listinfo/users
