[CP2K-user] DFT+U unphysical warning

[Nicholas Winner]

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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