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I tried to do geometry optimization for a N-doped nanographenes ( finite polycyclic aromatic hydrocarbon). I put a large molecule in a big box and ran with ISPIN=2 without any MAGMOM settings. Then I got the total mag as 0.4634. However for most of the N-doped nanographene with N substituting other carbon atoms the final mag is an integer. Now I am really confused. Is that right?

I have read some posts in this forum that for isolated molecule or cluster, the total magnetization should be a integer. Can anybody help me figure it out?

Here is the INCAR file
*************
SYSTEM = C53H20N (P1)
PREC = Accurate
ENCUT = 600
IBRION = 2
NSW = 100
ISIF = 2
ALGO = Normal (blocked Davidson)
NELM = 60
NELMIN = 2
EDIFF = 1.0e-05
EDIFFG = -0.01
VOSKOWN = 1
NBLOCK = 1
ISPIN = 2
INIWAV = 1
ISTART = 0
ICHARG = 2
LORBIT = 11
LWAVE = .FALSE.
LCHARG = .FALSE.
ADDGRID = .FALSE.
ISMEAR = 0
SIGMA = 0.05
LREAL = Auto
RWIGS = 0.77 0.32 0.75
*******************
Here is the KPOINTS setting:
*******************
Automatic mesh
0
Gamma
1 1 1
0. 0. 0.
*******************
PBE_PAW potentials
*******************

here is the POSCAR:
*******************
C53H20N (P1)
1.00000000000000
25.0000000000000000 0.0000000000000000 0.0000000000000000
0.0000004600000000 25.0000000000000000 0.0000000000000000
0.0000002200000000 0.0000002200000000 12.0000000000000000
53 20 1
Direct
0.3572565382964743 0.5005606403362663 0.4999964931244803
0.3856813510733432 0.4512290529415472 0.5000026457008034
0.4428532097672662 0.4516282910530319 0.5000039250332037
0.4710224341562534 0.5009957358394633 0.5000022108960706
0.4425095162733154 0.5502208944962126 0.4999982979674806
0.3853243519343899 0.5499791536641190 0.4999935569498566
0.4710099562539829 0.5998835221872767 0.4999991047188499
0.5283960307267586 0.5997615966824024 0.5000048797961266
0.5568221245603222 0.5499503332970015 0.5000024473542729
0.5283475255876879 0.5010281442970889 0.5000032137466619
0.2994233239366546 0.5002930561840319 0.4999945169862692
0.2718756452618212 0.5492887805638863 0.4999836388609988
0.2988659966801538 0.5986960634839559 0.4999760583959493
0.3564398223374803 0.5994373122518445 0.4999839057880879
0.3571784545573408 0.4016628496146182 0.5000064696843673
0.2995918158723837 0.4018676647911898 0.5000097988162305
0.2722359294181432 0.4510471659056259 0.5000041202057992
0.4713000768234696 0.4018677912132164 0.5000060369343614
0.4429145820568355 0.3527583594804403 0.5000044986269542
0.3852048370609671 0.3519804297897006 0.5000077101824076
0.5286580488496394 0.4027193697183296 0.5000060189023596
0.5568618829709444 0.4514431166646056 0.5000037960312933
0.6141803268606515 0.4513966922413915 0.4999921225792426
0.6425849439375394 0.5004751640097037 0.4999863303903818
0.6143078116779741 0.5499790085633842 0.4999974210306390
0.6139375975978892 0.3524180707506663 0.5000169964822097
0.6420750656786610 0.4018895871981897 0.4999943161061299
0.6430693184854416 0.5990580963189303 0.5000023264099747
0.7008344880761197 0.5981419955900472 0.4999929393847043
0.7278463872091028 0.5490470196368262 0.4999751541801433
0.7004428345045105 0.4999282329975455 0.4999713679633387
0.6998847331005622 0.4016238013200645 0.4999801607758437
0.7272383838164906 0.4504374932681779 0.4999654543572865
0.3841270576770742 0.6491808265795790 0.4999835337284562
0.3539699329925741 0.6967979939260793 0.4999665252021189
0.2980456854363147 0.6959392771915368 0.4999543745962699
0.2705757025579341 0.6477984571095502 0.4999645777390284
0.4423815762502117 0.6491707391124931 0.4999953755924499
0.5570674487108652 0.6489722336690963 0.5000124649572851
0.5273675824319725 0.6968105897217540 0.5000129806955581
0.4721322538506750 0.6969532105929535 0.5000035746490959
0.6154162827198580 0.6490981637770616 0.5000159001058864
0.7289910564107750 0.6475671080396330 0.5000026252807983
0.7015693263915418 0.6954832261349467 0.5000207632912415
0.6453848089288107 0.6964730597404568 0.5000276353448102
0.7270060058018787 0.3515040592141878 0.4999850615349500
0.2718960195973886 0.3524033720925734 0.5000175657736221
0.2997249548935291 0.3043686636687488 0.5000176034854815
0.3555724893091525 0.3038254698145426 0.5000124957712092
0.4740332233339041 0.3052493042371818 0.4999890833863304
0.5283682853058609 0.3069685871583657 0.4999907556645776
0.6984620371107159 0.3045182518823072 0.5000221154310308
0.6420227861684993 0.3041151456521812 0.5000397828927388
0.3740745719621357 0.7354361663634905 0.4999562368135683
0.2760382220791807 0.7336523862738568 0.4999402063572805
0.5480244353350650 0.7351726029081737 0.5000210193926822
0.4516472746684293 0.7354093539945596 0.5000042666333957
0.7235361384696989 0.7332285117515759 0.5000290332357351
0.6254366944408942 0.7351891840208634 0.5000417108850100
0.2281607608107871 0.5490342202797084 0.4999849259088049
0.2285255405641155 0.4509817246997758 0.5000075736370119
0.2269044676325841 0.6470172014054019 0.4999620745792952
0.7715566304014436 0.5489163237331623 0.4999671877383425
0.7709365356633870 0.4501068920881336 0.4999448070186105
0.7726685010330400 0.6468677791528882 0.4999945096031499
0.7706615410045686 0.3511894625481025 0.4999586617179449
0.2282193295825171 0.3527543405063747 0.5000207304457390
0.2778862737572858 0.2665614274308170 0.5000219011955697
0.3760583141550600 0.2653801858838086 0.5000155269106661
0.4553582614677882 0.2659600446938503 0.4999694473048855
0.5517838564303008 0.2705799974172806 0.4999688155816163
0.7194370474369640 0.2662223556936341 0.5000371198250407
0.6223000717936619 0.2654377286454938 0.5000845167243293
0.5568156700309778 0.3542398848430303 0.5000069990056011
****************

Thanks a lot.

This is a numerical effect caused by smearing. Smearing improves the convergency but can lead to such unphysical magnetic states. Tune SIGMA to smaller values and you will get magnetization 1.
BtW The calculation of a periodic slab is much cheaper than molecule in a large box.
[ Edited Fri Feb 22 2013, 11:50AM ]

It solved my problem. Thank you very much.