[CP2K-user] [CP2K:15178] DFT+U unphysical warning

[Matt W]

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 <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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>
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