Orbital moment of a single isolated Terbium (Tb) atom: LDA+U+SOC

[saurabh_2802]

Dear All,

I am doing a spin orbit calculation of a single isolated Tb atom (at the centre of a 16x16x16 A3 cell and the calculation has been done at Gama point)

I follow the usual procedure i.e. first I have done the spin polarized calculation and kept the self consistence CHGCAR and WAVECAR file, and then I start spin orbit calculation form these two files.

INCAR file is given as:

SYSTEM = Tb atom
ENCUT = 400.0
PREC = Normal
ISPIN = 2
MAGMOM = 0 0 6
ISTART = 1
ICHARG = 1
NELM = 80
NELMIN = 4
#NELMDL = -5
EDIFF = 0.0001
EDIFFG = -0.005
##############################
ISYM = 0
NSW = 0
#NBLOCK = 1
#KBLOCK = 0
IBRION = 1
POTIM = 0.1
ISIF = 2
IALGO = 48
LDIAG = .TRUE.
NSIM = 4
LPLANE= .TRUE.
#=TIME = 0.1
###################################
LDAU = .TRUE.
LDAUTYPE = 2
LDAUL = 3
LDAUU = 4.0
LDAUJ = 1.0
#LDAUPRINT=
LMAXMIX = 6
###################
LSORBIT = .TRUE.
SAXIS = 0 0 1
LORBMOM = .TRUE.
NBANDS = 32
################################
#RWIGS =
LORBIT = 11
#EMIN =
#EMAX =
ISMEAR = 0
SIGMA = 0.1
LREAL= Auto
#ROPT =
#NEDOS =
#ADDGRID = .TRUE.
#########################
#LWAVE = .FALSE.
#LCHARG = .FALSE.
#LVTOT = .FALSE.
#LELF = .FALSE.
#NPAR= 8
################################

POTCAR, KPOINTS and POSACR file is okay.. The calculation converged and the orbital momentum that I got is zero. But the value has to be 3 meuB. Although I am getting the correct spin moment, as given below.....
-------------------------------------------------------------------------
orbital moment (x)

# of ion p d f tot
------------------------------------------------
1 0.000 0.000 0.000 0.000
------------------------------------------------
0.000 0.000 0.000 0.000



orbital moment (y)

# of ion p d f tot
------------------------------------------------
1 0.000 0.000 0.000 0.000
------------------------------------------------
0.000 0.000 0.000 0.000



orbital moment (z)

# of ion p d f tot
------------------------------------------------
1 0.000 0.000 0.000 0.000
------------------------------------------------
0.000 0.000 0.000 0.000



total charge

# of ion s p d f tot
------------------------------------------------
1 2.908 6.032 0.000 9.428 18.368



magnetization (x)

# of ion s p d f tot
------------------------------------------------
1 0.000 0.000 0.000 0.000 0.000



magnetization (y)

# of ion s p d f tot
------------------------------------------------
1 0.000 0.000 0.000 0.000 0.000



magnetization (z)

# of ion s p d f tot
------------------------------------------------
1 0.072 -0.021 0.000 5.017 5.069

---------------------------------------------------------------
My question: Is my calculation right?? or I am doing something wrong?

Looking forward for your kind reply.

Thanking you and with regards,

Saurabh Ghosh

[boris]

Hi,

Starting from the CHGCAR and WAVECAR files without SOC is not an appropriate method. I know that it is the method described in the vasp manual, but unfortunately it does not work.

Your calculation results show that the orbital moment is zero. The reason for this is that the nondiagonal spin components, that give rise to the orbital moment, are probably empty. You can check this by setting LDAUPRINT = 1. This will allow vasp to print the occupation matrix of the Tb f shells at the end of the OUTCAR file. There should be 8 occupation matrices (4 spin components, each with a real and imaginary part) with only the 1st and 4th spin components filled. The 2nd and 3rd spin components should be empty, hence no orbital moment.

I guess the right way of doing this is not to use the CHGCAR and WAVECAR files and to start directly the calculation with SOC (ISTART=0, ICHARG=2).

Also, you should use a non cubic supercell, something like 15x16x17 A3. This will break all symmetries in the system and will allow the calculation to reach the ground state more easily.

Finally, beware of the metastable states created by the LDA+U formalism. With SOC, it will be even worse.

Regards