[CP2K-user] [CP2K:11881] DFT+U with Ti and O

[Krack Matthias (PSI)]

Hi Kazimieras

You will get that warning whenever you have more than six 2p electrons for O or two for the O 2s, though I assume that you want to have a Hubbard U correction for the Ti 3d states alone or for both Ti 3d and O 2p states. Other choices might not made much sense.
Mulliken is known to give orbital occupations which can be slightly larger than the limit of 2l+1 (or 4l+2 for closed shell) for some cases, e.g. more than 10 d electrons for Zn. You can ignore that warning if the occupation is only slightly larger than expected. There is a print key https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/PRINT/PLUS_U.html which prints the occupations of the orbitals for which a U value greater zero has been assigned in the input.

Matthias

From: cp... at googlegroups.com <cp... at googlegroups.com> On Behalf Of kazimieras tamoliunas
Sent: Montag, 17. Juni 2019 09:46
To: cp2k <cp... at googlegroups.com>
Subject: [CP2K:11881] DFT+U with Ti and O

Hello,
 i'm calculating Na(+1)Ti2(+4)(PO4)3(-3) using DFT+U method and the system is quite big ( 108 atoms, calculating 2x2x1, so 432 atoms in total) and i'm trying to fit the U parameter. Now i'm using U for both Ti and O (obviously Ti d orbitals). However, what's less obvious is what kind of orbitals should i apply the U correction to, regarding oxygen? Whenever i apply L=0 or L=1 ( p sounds reasonable to me) it prints out a following message:

*** 04:19:11 WARNING in dft_plus_u:mulliken :: DFT+U energy contibution ***
 *** is negative possibly due to unphysical Mulliken charges. Check your ***
 *** input, if this warning persists or try a different method!

Now when i set L=2 for O, the warning disappears and i get a great convergence.
That i do like, however, i cannot understand how this method works. I suppose i have oxygen mostly -2, therefore it should have it's 2p fully filled. From my point of view, if i want to localise the electron i should think about 3s or 2p, but certainly not 3d orbital. Can somebody help me with that?
Appreciate any help, comments or possible literature from the ones familiar with the problem!

PS. i know that these oxidation states are not correct and you cannot precisely have them in a periodic system. However, i suppose, that DFT+U method is used for certain electron localisation, therefore, i feel that they provide some ground to rely upon. Correct me if i'm mistaken!!

Regards,
Kazimieras
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