[CP2K-user] [CP2K:15205] DFT+U unphysical warning

[Krack Matthias (PSI)]

Nick, we are talking about dense solid state systems. For such system, space is already well covered by a DZV basis set at each atom. The situation is, however, different already for molecular liquids like water or for systems with larger voids (MOFs, surfaces) or even small isolated molecular systems. Soft functions are required to describe the electron density decaying into the void regions. More polarisation function (second set of d and a set of functions) certainly improve the description, but  these function sets increase the computational costs significantly while their impact is rather moderate for DFT applied for O in dense solid state systems. I tried TZV/QZV and/or more polarisation functions for O in such systems, but the gain was rather small compared to the additional computational costs. There are tutorials<https://www.cp2k.org/exercises:2015_cecam_tutorial:basis_set_optimisation_using_optimize_basis?s%5b%5d=molopt&s%5b%5d=na> showing how you may generate new or augment existing MOLOPT basis sets. It is also important to employ a balanced set of basis sets for a system which is, of course, also true for other system types. A mixing of SR and non-SR basis sets should always be done with care. The optimisation of the SR basis sets includes also the condition number of the overlap matrix as a parameter during the basis set generation procedure which allows for a control of its numerical stability.

Matthias

Von: cp... at googlegroups.com <cp... at googlegroups.com> Im Auftrag von Nicholas Winner
Gesendet: Donnerstag, 22. April 2021 20:34
An: cp2k <cp... at googlegroups.com>
Betreff: Re: [CP2K:15205] DFT+U unphysical warning


Matthias, after investigating further, it does seem like using the SR potential is fairly consistent in giving good results for the DFT+U calculations, even when Mulliken leads to a negative +U energy. I have previously not used DFT+U, and always used TZV2PX for O finding no problems, but the population analysis does seem to require a better conditioned overlap matrix.

Matt mentioned TZV-SR basis sets in the UCL file, which is true, but not for O. In fact there are a few elements which only go up to DZVP-SR across all the BASIIS_MOLOPT_{, UCL, LnPP1} files. Some of these are O, C, Cl, F, H, etc. which are very common elements. H for instance only has SZV-SR and the rest are non-SR. If I believe that SR should always be used for stability in condensed phase, then it seems like these need some new basis sets, at least up to TZV quality. Maybe you disagree and DZVP is sufficient for these elements for some reason, while others really need larger bases?

Is there a reference for the procedure for fitting the SR basis sets? The molopt sets are described in "Joost VandeVondele and Juerg Hutter, J. Chem. Phys. 127, 114105 (2007)", but the BASIS_MOLOPT file simply says "variants of these basis sets using less and thus less diffuse primitives " were used for the SR versions, which is a little bit vague to me.
On Thursday, April 22, 2021 at 2:18:31 AM UTC-7 Matt W wrote:
Just to note there are MOLOPT-TZVP-SR in the BASIS_MOLOPT_UCL files distributed with CP2K.
Matt
On Wednesday, April 21, 2021 at 11:58:19 PM UTC+1 n... at berkeley.edu wrote:
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<mailto:c... at googlegroups.com> <c... at googlegroups.com<mailto:c... at googlegroups.com>> Im Auftrag von Nicholas Winner
Gesendet: Mittwoch, 21. April 2021 19:36
An: cp2k <c... at googlegroups.com<mailto: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<mailto:c... at googlegroups.com> <c... at googlegroups.com<mailto:c... at googlegroups.com>> Im Auftrag von Nicholas Winner
Gesendet: Mittwoch, 21. April 2021 19:15
An: cp2k <c... at googlegroups.com<mailto: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<mailto:c... at googlegroups.com> <c... at googlegroups.com<mailto:c... at googlegroups.com>> Im Auftrag von Nicholas Winner
Gesendet: Freitag, 16. April 2021 22:57
An: cp2k <c... at googlegroups.com<mailto: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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