[CP2K-user] [CP2K:15176] DFT+U unphysical warning

[Nicholas Winner]

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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