[CP2K:9422] DFT+U Calculation

[Krack Matthias (PSI)]

Dear Sudhir

This warning pops up when the Mulliken population analysis (MPA) returns an orbital occupation larger than the maximum number of electrons allowed for the actual l value specified in the +U section, e.g. more than 10 electrons are assigned by the MPA to the Cu 3d (l=2) orbitals. This can be an indicator for a too large U value enforcing a too high on-site localisation. Please, note that the U(eff) values successfully employed by PW codes are not transferable to CP2K. The corresponding U values for CP2K are usually smaller, roughly by a factor of two, e.g. with respect to their impact on the band gap. You can specifically print the detailed orbital occupations of the +U atoms using this CP2K print key<https://manual.cp2k.org/cp2k-4_1-branch/CP2K_INPUT/FORCE_EVAL/DFT/PRINT/PLUS_U.html> with “QS_SCF 1” in its &EACH section. The warning appears easily for systems like ZnO with a 3d10 configuration when the MPA assigns for instance 10.04 electrons to the Zn 3d orbitals of some Zn atoms. The warning can then be ignored possibly for such spurious violations of the maximum orbital occupation. It is up to the user eventually to decide, if the obtained results are still physically meaningful. There is always the option to employ another method when DFT+U troubles, e.g. DFT with hybrid functionals.

Best regards

Matthias

From: cp... at googlegroups.com [mailto:cp... at googlegroups.com] On Behalf Of sudhir kumar Sahoo
Sent: 18 September 2017 15:03
To: cp2k
Subject: [CP2K:9422] DFT+U Calculation

Dear cp2k users,

I am using DFT+U method for modelling CuInSe2 (U=5.0 eV for 3d electrons of Cu), particularly, using Mulliken method as implemented in cp2k program.
However, a warning is repeatedly written in the output file.

 *** WARNING in dft_plus_u.F:1455 :: DFT+U energy contibution is negative ***
 *** possibly due to unphysical Mulliken charges. Check your input, if    ***
 *** this warning persists or try a different method!                     ***

After reading the cp2k google group discussion, I have tried to perform DFT+U calculations as explained below,
but each time I got the same warning in the output file.

1) Restarting the calculation U=5 eV, from an optimized wavefunction obtained from U=0 eV.
2) Point 1 + u_ramping option
3) Point 2 + &enforce_occupation

Can someone please help me in this.

Thanking You,
Sudhir

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