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hello:
I'm calculate a system contain a Fe atom. I doubled the cell for FM and AFM calculations, for FM, i can get local moment about 2 for each Fe atoms. but for AFM, the local moment of both Fe atoms are 0! it supposed to be about 2 and -2, what shoul i do?
any advise will be appriciated!

You provide too sparse information in order to say something concrete, but please

1) Make sure you have initiated the MAGMOM tag suffiently high. Typically 1.2 or 1.5 times the supposed converged value is a good guess. If you initiate MAGMOM too low the calculation may converge to another magnetic solution instead, such as the nonmagnetic one.

2) Make sure you have supplied the correct volume. If you compress the system too much, there is no AFM solution for Fe.

Cheers,
/Dan

Dear Dan:
Thank you for your replay!

1) the calculated FM moment is 2.001, so in AFM calculation I set MAGMOM to be 2.5 and -2.5 for Fe atoms.

2) the colume is correct, i haven't compress the system

the problem is solved by using ISYM=-1, then the AFM local moment of Fe atoms become 1.999 -1.999, so why? for other AFM calculations I didn't use ISYM=-1 there is no problem about local moment. I think I read somewhere that VASP can recognize the symmetry breaking of initial MAGMOM set.

It is possible to calculate the AFM solution for Fe without changing ISYM. You have missed something in your cell setup if you can't obtain this solution. Please post your input files (INCAR, POSCAR, KPOINTS) and I will have a look at them.

/Dan

Dear Dan:
sorry for reply so late, follow are my input files.
#####################INCAR#################

ISTART = 0
#ICHARG = 1

ENCUT = 500
#ENAUG = 606

IBRION = -1
NSW = 0
#EDIFFG = -0.01
#POTIM = 5

#ISMEAR = -5
#ALGO = Fast

PREC = High
EDIFF = .1E-4

ISPIN = 2
#LSORBIT = T
#SAXIS=0 0 1
MAGMOM = 64*0 8 -8

#LORBMOM = T
VOSKOWN = 1
GGA_COMPAT = F

LMAXMIX=4
LORBIT=11

LWAVE = F

#############POSCAR#################
4x4C-Fe AFM
1.000000000000000
17.043372000000002 9.839995999999998 0.000000000000000
0.000000000000000 9.839995999999998 0.000000000000000
0.000000000000000 0.000000000000000 18.000000000000000
64 2
Direct
0.0000000000000000 0.0000000000000000 -0.0001086111111111
0.4999999999999999 0.0000000000000001 -0.0001086111111111
0.0000000000000000 0.2499999999999999 -0.0001086111111111
0.4999999999999999 0.2500000000000001 -0.0001086111111111
0.0000000000000000 0.4999999999999999 0.0000471111111111
0.4999999999999999 0.5000000000000001 0.0000471111111111
0.0000000000000000 0.7500000000000000 0.0000471111111111
0.4999999999999999 0.7500000000000002 0.0000471111111111
0.1250000293369175 -0.0000000801499463 -0.0001086111111111
0.6250000293369175 -0.0000000801499462 -0.0001086111111111
0.1250000293369175 0.2499999198500536 0.0003568333333333
0.6250000293369175 0.2499999198500537 0.0003568333333333
0.1250000293369175 0.4999999198500536 -0.0002379444444444
0.6250000293369175 0.4999999198500538 -0.0002379444444444
0.1250000293369175 0.7499999198500537 0.0003568333333333
0.6250000293369175 0.7499999198500537 0.0003568333333333
0.2499999999999999 0.0000000000000000 0.0000471111111111
0.7499999999999997 0.0000000000000002 0.0000471111111111
0.2499999999999999 0.2500000000000001 -0.0002379444444444
0.7499999999999997 0.2500000000000003 -0.0002379444444444
0.2499999999999999 0.5000000000000001 -0.0002379444444444
0.7499999999999997 0.5000000000000003 -0.0002379444444444
0.2499999999999999 0.7500000000000000 0.0000471111111111
0.7499999999999997 0.7500000000000000 0.0000471111111111
0.3750000293369175 -0.0000000801499464 0.0000471111111111
0.8750000293369171 -0.0000000801499460 0.0000471111111111
0.3750000293369175 0.2499999198500537 0.0003568333333333
0.8750000293369171 0.2499999198500539 0.0003568333333333
0.3750000293369175 0.4999999198500537 0.0000471111111111
0.8750000293369171 0.4999999198500539 0.0000471111111111
0.3750000293369175 0.7499999198500538 -0.0001528888888889
0.8750000293369171 0.7499999198500540 -0.0001528888888889
0.0416666960035843 0.0833332531833869 -0.0001528888888889
0.5416666960035842 0.0833332531833870 -0.0001528888888889
0.0416666960035843 0.3333332531833869 0.0000471111111111
0.5416666960035842 0.3333332531833870 0.0000471111111111
0.0416666960035843 0.5833332531833868 0.0003568333333333
0.5416666960035842 0.5833332531833870 0.0003568333333333
0.0416666960035843 0.8333332531833869 0.0000471111111111
0.5416666960035842 0.8333332531833869 0.0000471111111111
0.1666666666666665 0.0833333333333334 0.0000471111111111
0.6666666666666664 0.0833333333333335 0.0000471111111111
0.1666666666666665 0.3333333333333334 -0.0002379444444444
0.6666666666666664 0.3333333333333336 -0.0002379444444444
0.1666666666666665 0.5833333333333335 -0.0002379444444444
0.6666666666666664 0.5833333333333335 -0.0002379444444444
0.1666666666666665 0.8333333333333334 0.0000471111111111
0.6666666666666664 0.8333333333333335 0.0000471111111111
0.2916666960035841 0.0833332531833871 0.0003568333333333
0.7916666960035840 0.0833332531833872 0.0003568333333333
0.2916666960035841 0.3333332531833870 -0.0002379444444444
0.7916666960035840 0.3333332531833872 -0.0002379444444444
0.2916666960035841 0.5833332531833871 0.0003568333333333
0.7916666960035840 0.5833332531833872 0.0003568333333333
0.2916666960035841 0.8333332531833871 -0.0001086111111111
0.7916666960035840 0.8333332531833871 -0.0001086111111111
0.4166666666666665 0.0833333333333335 0.0000471111111111
0.9166666666666664 0.0833333333333336 0.0000471111111111
0.4166666666666665 0.3333333333333335 0.0000471111111111
0.9166666666666664 0.3333333333333337 0.0000471111111111
0.4166666666666665 0.5833333333333334 -0.0001086111111111
0.9166666666666664 0.5833333333333336 -0.0001086111111111
0.4166666666666665 0.8333333333333334 -0.0001086111111111
0.9166666666666664 0.8333333333333336 -0.0001086111111111
0.2083333626702508 0.4166665865167203 0.0864383333333333
0.7083333626702507 0.4166665865167205 0.0864383333333333

##################KPOINTS################
auto
0
Gamma
4 7 1
0 0 0

thank you, Dan

I would recommend you to be more specific next time about your system. From your statement above "I'm calculate a system contain a Fe atom. I doubled the cell for FM and AFM calculations" I assumed you were talking about a bulk Fe system.

For your current system, I do not really have an answer at the moment without more details. Try to see if changing SYMPREC can help to determine another symmetry from the start, in order to see if it's a symmetry issue. It may also be a convergence issue, so try to see if changing the mixing parameters can help or switch the electronic algorithm.


<span class='smallblacktext'>[ Edited Sun May 09 2010, 09:02PM ]</span>