strange occupation

Message

deltz · #1 Post by **deltz** » Mon Oct 20, 2008 8:45 am

Dear vasp users,
i my case, c59N@CNNT, i found the occupation is very strange, at Gamma point it appears as follows, and the fermi energy is -1.8675 eV, in my opinion, the occupation number at band 883 and 884 should be 2.00, and those in band 886 should be 0.00, i have increased the cal. accuracy by using a denser k-grid, but this problem remains, by the way, icalulations of CNNT, the occupation is allright, my question is, how to explain this unusual occupation numbers?

best regards
Dai

873 -3.0936 2.00000
874 -3.0025 2.00000
875 -2.9827 2.00000
876 -2.9010 2.00000
877 -2.8088 2.00000
878 -2.7814 2.00000
879 -2.6892 2.00000
880 -2.6477 2.00000
881 -2.4123 2.00000
882 -2.2017 2.00000
883 -1.9568 1.79351
884 -1.9460 1.73319
885 -1.8680 1.00566
886 -1.8461 0.76187
887 -1.5992 0.00015
888 -1.5683 0.00002

#2 Post by **admin** » Mon Oct 20, 2008 11:40 am

please give more details of your calculation (INCAR, POSCAR, KPOINTS) . the occupation depends on the BZ integration method and the smearing width you use, so from the data you show nothing can be said. Please note hat the numberof k-points in the directions perpendicular to the tube has to be 1 (I hope you have kept that in mind when you increased the mesh).

deltz · #3 Post by **deltz** » Tue Oct 21, 2008 10:28 am

Dear admin :
thank you for your answer!
my INCAR is as follows:
SYSTEM=C N
IALGO=38
NELM=200
NELMIN=4
EDIFF=1E-04
ISMEAR=0
SIGMA=0.1
LREAL=Auto
ISYM=0
IBRION=-1
PREC=normal
LWAVE=.FALSE.
LCHARG=.TRUE.

KPOINTs file is:
Auto
0
Monk-horst
3 3 3
0.0 0.0 0.0

POSCAR:
SYS=C N
1.00000000000000
23.0000000000000000 0.0000000000000000 0.0000000000000000
0.0000000000000000 23.0000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 25.6950000000000003
441 1
Direct
........

when I change KPONTS file to :

Auto
0
Monk-horst
1 1 5
0.0 0.0 0.0

the occupation at Gamma point becomes:
881 -2.4128 2.00000
882 -2.2018 2.00000
883 -1.9568 1.79484
884 -1.9461 1.73569
885 -1.8697 1.02799
886 -1.8459 0.76284
887 -1.5990 0.00015
888 -1.5681 0.00002

it already has the problem I mentioned above.

#4 Post by **admin** » Wed Oct 22, 2008 9:19 am

1) your first k-point set (3x3x3) was obviously wrong.
2) for the second run,
-- SIGMA=0.1 is rather high, please reduce it to 0.01 or even lower.
-- please check if the results are converged. 10-4 might be not enough to give well-converged results, please increase to 1e-6.

deltz · #5 Post by **deltz** » Thu Oct 23, 2008 8:42 am

Thank you very much for your reply,I will have a try.

#6 Post by **admin** » Thu Oct 23, 2008 10:20 am

all the best! I realize that your system is rather large, and the small SIGMA might slow down convergence....