Effect of +U on energy of free atom
Matthias Krack
matthia... at psi.ch
Fri Oct 14 10:23:31 UTC 2016
Hi
indeed the +U energy should be always positive. A negative value indicates
that the applied population analysis returns unreasonable d orbital
occupations which can happen e.g. for Cu when slightly more than 10
electrons are assigned to the 3d orbitals by the Mulliken analysis. That is
a known shortcoming of the Mulliken analysis. Lowdin is more robust in that
respect, but there are no forces implemented for Lowdin in CP2K.
Furthermore, I do not recommend to use the MULLIKEN_CHARGES method. So the
only reasonable choice for the PLUS_U_METHOD is currently MULLIKEN. Note
that PW codes like VASP use different atomic basis sets.for the
determination of the orbital occupations which results in different orbital
occupations and thus the U values are not transferable to CP2K.
Matthias
On Thursday, 13 October 2016 18:19:36 UTC+2, Satish Kumar wrote:
>
> Hello CP2K users
>
> I wanted to understand what is the effect of adding +U correction to the
> energies of calculations without any U. For example, I have a Cu atom for
> which I am interested in finding the DFT and DFT+U energy. I have provided
> the input file below.
>
> In an attempt to see what U does, I looked at the Fermi energies of the
> two calculations. When I do an energy run without any U, I get the Fermi
> energy to be -0.163318 Ha while upon inclusion of U=5.0 eV, I get the Fermi
> energy to be -0.16497 Ha. The difference (Energy with U - Energy without U)
> is -0.05 eV. This means that adding U shifts the energy levels of the
> electronic states to more negative values. I tried the same calculation
> with VASP and I get the same difference in fermi energies to be +0.12 eV. I
> was thinking that adding U would shift the fermi energy to more positive
> values compared to when no U is applied. Is that correct?
>
> Secondly, with respect to the DFT energy of a Cu atom, in CP2K, the effect
> of U is negligible. Adding U =5.0 eV gives a "DFT+U energy:
> 0.00000001917427", which is almost zero. However, Using VASP, when U=5.0
> eV was turned on, the DFT energy of Cu atom becomes more positive by +1.2
> eV compared to calculation without U. Could someone help me with what is
> going on? or if there are some changes to the input file that may fix this
> issue of almost zero DFT+U energy?
>
> On a different note, I was thinking that since Cu atom has all the d
> electrons filled and paired up, application of U to these d electrons would
> not change the electronic occupations and hence would not change the DFT+U
> energy. But from VASP it did look like the electronic occupations were not
> changed much by inclusion of U but the energies did change considerable. I
> was confused what is going on.
>
> Finally, I used PLUS_U_METHOD LOWDIN since MULLIKEN/MULLIKEN_CHARGES gave
> me the following warnings. Does this change the above things by any chance?
> I don't think so. But if it does, please let me know any suggestions to get
> rid of the warning.
>
> *** 13:28:53 WARNING in dft_plus_u:mulliken_charges :: DFT+U energy
> ***
>
> *** contibution is negative possibly due to unphysical Mulliken charges.
> ***
>
> *** Check your input, if this warning persists or try a different method!
> ***
>
>
> Input file:
>
>
> &FORCE_EVAL
>
> METHOD Quickstep
>
> &DFT
>
> PLUS_U_METHOD LOWDIN
>
> UKS
>
> BASIS_SET_FILE_NAME /home/siyemperumal/GTH_BASIS_SETS_5-12-10
>
> POTENTIAL_FILE_NAME /home/siyemperumal/GTH_POTENTIALS_5-12-10
>
> WFN_RESTART_FILE_NAME mol-RESTART.wfn
>
> &MGRID
>
> CUTOFF 300
>
> &END MGRID
>
> &QS
>
> WF_INTERPOLATION ASPC
>
> EXTRAPOLATION_ORDER 3
>
> &END QS
>
> &SCF
>
> EPS_SCF 1.E-6
>
> SCF_GUESS RESTART
>
> MAX_SCF 500
>
> &DIAGONALIZATION
>
> ALGORITHM STANDARD
>
> &END DIAGONALIZATION
>
> ADDED_MOS 60
>
> &SMEAR ON
>
> METHOD FERMI_DIRAC
>
> ELECTRONIC_TEMPERATURE [K] 300
>
> &END SMEAR
>
> &MIXING
>
> METHOD DIRECT_P_MIXING
>
> &END MIXING
>
> &END SCF
>
> &XC
>
> &XC_FUNCTIONAL PBE
>
> &END XC_FUNCTIONAL
>
> &VDW_POTENTIAL
>
> DISPERSION_FUNCTIONAL PAIR_POTENTIAL
>
> &PAIR_POTENTIAL
>
> TYPE DFTD3(BJ)
>
> PARAMETER_FILE_NAME
> /home/siyemperumal/Research/cp2k/package/cp2k/data/dftd3.dat
>
> REFERENCE_FUNCTIONAL PBE
>
> &PRINT_DFTD MEDIUM
>
> &END PRINT_DFTD
>
> &END PAIR_POTENTIAL
>
> &END VDW_POTENTIAL
>
> &END XC
>
> &PRINT
>
> &MO
>
> EIGENVALUES
>
> OCCUPATION_NUMBERS
>
> &EACH
>
> QS_SCF 0
>
> &END EACH
>
> &END MO
>
> &END PRINT
>
> &END DFT
>
> &SUBSYS
>
> &CELL
>
> ABC 12.00 13.00 14.00
>
> PERIODIC NONE
>
> &END CELL
>
> &COORD
>
> Cu 0.0000153980 0.0000010425 0.0006891492
>
> &END COORD
>
> &KIND Cu
>
> BASIS_SET DZVP-MOLOPT-SR-GTH
>
> POTENTIAL GTH-PBE-q11
>
> #I remove the following DFT calculation. I also tried ramping and other
> associated keywords but with no success.
>
> &DFT_PLUS_U
>
> L 2
>
> U_MINUS_J [eV] 5.00
>
> &END DFT_PLUS_U
>
> &END SUBSYS
>
> &END FORCE_EVAL
>
> &GLOBAL
>
> PROJECT mol
>
> RUN_TYPE ENERGY
>
> PRINT_LEVEL LOW
>
> &END GLOBAL
>
>
>
>
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