[CP2K-user] [CP2K:15181] DFT+U unphysical warning

[Krack Matthias (PSI)]

Hello Nick

The O DZVP-MOLOPT-SR basis set is accurate for condensed phase systems. If you want to explore the basis set limit for such systems, then you better use a plane waves code. You can add, of course, further polarization functions, but softer basis functions, as they are included in basis sets for molecular systems, will inevitably cause problems due to over-completeness which will result in ill-conditioned overlap matrices. The Mulliken orbital charges calculated with such overlap matrices are usually not reasonable and thus the calculation of the +U contribution is spoiled. The Löwdin analysis is more resilient than the Mulliken analysis in that respect, but it does not solve the basic problem. I am surprised that the restart using a TZV2PX basis set from a DZVP wfn restart file worked. That sounds more like garbage-in garbage-out.

Matthias

Von: cp... at googlegroups.com <cp... at googlegroups.com> Im Auftrag von Nicholas Winner
Gesendet: Donnerstag, 22. April 2021 00:58
An: cp2k <cp... at googlegroups.com>
Betreff: Re: [CP2K:15181] DFT+U unphysical warning

So something very strange happened.

I'm hesitant to switch O to a SR basis, because only SZV/DZVP are available. SZV isn't production quality and DZVP really isn't near the basis set limit, but I gave it a try to see what would happen. The warning about negative DFT+U persists, but the energy changed to  -15383.653493175701442. I then re-initialized with TZV2PX and converged to  -15385.210534637011733.

So to summarize I have three values for U=1.75
-15378.643602015281431 with TZV2PX
-15383.653493175701442 with DZVP
-15385.210534637011733 after restarting TZV2PX from the DZVP wfn.

These are enormous differences in energy and I don't know what to make of it. Any more ideas about what is going wrong based on my inputs?

On Wednesday, April 21, 2021 at 11:12:25 AM UTC-7 Matthias Krack wrote:
At a first glance, the basis set TZV2PX-MOLOPT-GTH-q6 for O is not well suited for condensed phase systems. You should rather use MOLOPT basis sets with “SR” in the name.

HTH

Matthias

Von: c... at googlegroups.com <c... at googlegroups.com> Im Auftrag von Nicholas Winner
Gesendet: Mittwoch, 21. April 2021 19:36
An: cp2k <c... at googlegroups.com>
Betreff: Re: [CP2K:15178] DFT+U unphysical warning

Here is my input file for Mn3O4.
On Wednesday, April 21, 2021 at 10:30:13 AM UTC-7 Matthias Krack wrote:
Hello Nick

The selection of the initial guess can help to achieve convergence. Could you provide a case which fails to converge with MULLIKEN? Otherwise, it is difficult to give further hints.

Matthias

Von: c... at googlegroups.com <c... at googlegroups.com> Im Auftrag von Nicholas Winner
Gesendet: Mittwoch, 21. April 2021 19:15
An: cp2k <c... at googlegroups.com>
Betreff: Re: [CP2K:15176] DFT+U unphysical warning

Thanks for the reply Matthias. I've had moderate success following what you suggested.

One issue that still persists, however, is that for some calculations I can only get convergence by using PLUS_U_METHOD LOWDIN. For example, MnO with Ueff=1.75eV, diagonalization and small smearing (T=100). I've tried using diagonalization and OT with MULLIKEN to get convergence, but sometimes LOWDIN seems to be the only way. Unfortunately, Lowdin does not support forces, so relaxations are out of the question.

Do you have a suggestion for how to work around this?

-Nick
On Friday, April 16, 2021 at 11:00:09 PM UTC-7 Matthias Krack wrote:
Hello Nick

I do not recommend the use of the PLUS_U_METHOD MULLIKEN_CHARGES. Just use the PLUS_U_METHOD MULLIKEN. The Mulliken population analysis can give unphysical orbital occupations, i.e. values greater than one (UKS case) or two (RKS case). Often the maximum occupation is only slightly exceeded and the warning can be ignored safely. You can print the orbital occupations for the orbitals affected by the +U correction with this print key<https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL/DFT/PRINT/PLUS_U.html> at the PRINT_LEVEL medium to check the actual occupation. Note, that the U values found appropriate with PW codes in the literature are not necessarily optimal for CP2K, too. CP2K often gives a similar effect, e.g. impact on the band gap, for smaller U values.

HTH

Matthias

Von: c... at googlegroups.com <c... at googlegroups.com> Im Auftrag von Nicholas Winner
Gesendet: Freitag, 16. April 2021 22:57
An: cp2k <c... at googlegroups.com>
Betreff: [CP2K:15147] DFT+U unphysical warning


Hello all, I am running some DFT+U calculations on Mn-O systems. While I have found a cp2k effective U value in the literature of ~1.3 eV for Mn. I notice that when tuning the value myself, I begin seeing the following warning once the U value reaches 0.5eV.

*** WARNING in dft_plus_u.F:2006 :: DFT+U energy contibution is negative ***

 *** possibly due to unphysical Mulliken charges!

Now this is only a warning, not an indication that the calculation is *necessarily* wrong, but it is troubling at least. Especially when my U value is nowhere near the size of lit value. I am using PLUS_U_METHOD MULLIKEN_CHARGES in order to have a marginally more robust solution. Does anyone have experience with how seriously to take this warning? I don't have a frame of reference to know if I should ignore it.



-Nick
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