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
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 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
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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
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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
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Hello Nick
The O DZVP-MOLOPT-SR basis set is accurate for condensed phase systems. If you want to explore the basis set limit for such systems, then you better use a plane waves code. You can add, of course, further polarization functions, but softer basis functions, as they are included in basis sets for molecular systems, will inevitably cause problems due to over-completeness which will result in ill-conditioned overlap matrices. The Mulliken orbital charges calculated with such overlap matrices are usually not reasonable and thus the calculation of the +U contribution is spoiled. The Löwdin analysis is more resilient than the Mulliken analysis in that respect, but it does not solve the basic problem. I am surprised that the restart using a TZV2PX basis set from a DZVP wfn restart file worked. That sounds more like garbage-in garbage-out.
Matthias
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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 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.
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