Doubt on Running a DFT+U calculation successfully!!

Message

muthukumar2k3 · #1 Post by **muthukumar2k3** » Thu Jun 17, 2010 10:34 pm

Dear Colleague,

I have a doubt in starting a DFT+U calculation.
I created my INCAR, but i donot know, if i have accounted all the parameters in my file that are necessary for the 'DFT+U' computation.

My system has one Lanthanoid element, and i wish to make a DFT+U calculation for f orbitals.

I attach here the following INCAR file, and added the
DFT+U parameters at the end.

---------------------------------------------------------
Startparameter for this Run:
NWRITE = 2
LPETIM = F write-flag & timer

Electronic Relaxation 1
PREC = normal
ENMAX = 500.00 eV
NELM = 40
EDIFF = 1E-04 stopping-criterion for ELM
MAXMIX = 40

Ionic Relaxation
EDIFFG = -0.01 stopping-criterion for IOM
NSW = 100 of steps for IOM
NBLOCK = 1; KBLOCK = 100 inner block; outer block
IBRION = 1 ionic relax: 0-MD 1-quasi-New 2-CG
ISIF = 2 stress and relaxation
IWAVPR = 11 prediction: 0-non 1-charg 2-wave 3-comb
ISYM = 1 0-nonsym 1-usesym
LCORR = T Harris-correction to forces
ISMEAR = 0
ISPIN = 2
NUPDOWN = 2
LREAL= Auto

DFT+U Computation
LDAU=.TRUE
LDAUTYPE= 2
LDAUL= 3
LDAUU=??
LDAUJ=??
---------------------------------------------------------

I understand that the total energy of the system, depends upon both the U and J values. But i dont know, how i should start with the values.

Does this goes from say 1 to n (with the point 'n' and 'n+1' giving the same energy) and then taking 'n' value as the U value (similarly for j).

I believe there should be something that makes my life easier. Could anyone help me sorting this issue out.

boris · #2 Post by **boris** » Tue Jun 22, 2010 9:09 am

Hi

The problem with LDA+U is that you never know what U and J to apply. Sometimes, the values of U and J are determined based on experimental results. But in general, you have to try several different values of U and J and check if you correctly predict physical quantities, such as bulk modulus, elastic constants, bandgap, etc.

In your case, it is an isolated atom. So difficult to tell what should be the U and J values.

muthukumar2k3 · #3 Post by **muthukumar2k3** » Wed Jun 23, 2010 4:28 pm

Hi Boris,
Many thanks for your reply.
Which experiment helps to determine the U and J values.

boris · #4 Post by **boris** » Thu Jun 24, 2010 7:27 am

Hi
X-Ray Photoemission spectra can be analyzed to determine the U and J values. This has been done in actinide and rare-earth oxides [A. Kotani and T. Yamazaki, Progress of Theoretical Physics Supplement No. 108, 1992].

You can also check the literature to see if someone has previously made a DFT+U calculation on your system and see what values of U and J he has used.

Regards