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I have a problem to run vasp for Fe slab 15 layers. I could successfully run 5 and 11 layers. Attached below are my POSCAR, INCAR, and the beginning of failed vasp log file. I appreciate if anyone could help me out. Thank you very much.

Tak

POSCAR:

bcc Fe
2.866
1.0 0.0 0.0
0.0 1.0 0.0
0.0 0.0 22.0
15
Cartesian
0.0 0.0 0.0
0.5 0.5 0.5
0.0 0.0 1.0
0.5 0.5 1.5
0.0 0.0 2.0
0.5 0.5 2.5
0.0 0.0 3.0
0.5 0.5 3.5
0.0 0.0 4.0
0.5 0.5 4.5
0.0 0.0 5.0
0.5 0.5 5.5
0.0 0.0 6.0
0.5 0.5 6.5
0.0 0.0 7.0

INCAR:

SYSTEM = Fe bcc(100)
ISTART = 0
ISMEAR = 2
SIGMA = 0.2
ENCUT = 350

ISPIN = 2
MAGMOM = 15*3

IBRION = 2
ISIF = 3
NSW = 15
EDIFF = 1.0E-05

Beginning of output

N E dE d eps ncg rms rms(c)
DAV: 1 0.802423707316E+03 0.80242E+03 -0.39552E+04 7008 0.965E+02
DAV: 2 -0.623663566906E+02 -0.86479E+03 -0.79773E+03 7000 0.256E+02
DAV: 3 -0.133579309957E+03 -0.71213E+02 -0.65342E+02 7352 0.108E+02
DAV: 4 -0.135349855661E+03 -0.17705E+01 -0.17579E+01 7544 0.204E+01
DAV: 5 -0.135372496884E+03 -0.22641E-01 -0.22637E-01 7768 0.185E+00 0.310E+01
DAV: 6 -0.566255213600E+03 -0.43088E+03 -0.12569E+03 13360 0.288E+02 0.125E+02
DAV: 7 -0.113645523331E+03 0.45261E+03 -0.33052E+03 12032 0.129E+02 0.698E+01
DAV: 8 -0.133438739209E+03 -0.19793E+02 -0.31420E+02 7576 0.135E+02 0.651E+01
DAV: 9 -0.174335823759E+03 -0.40897E+02 -0.98040E+01 9472 0.106E+02 0.812E+01
DAV: 10 -0.215001617334E+03 -0.40666E+02 -0.12251E+02 9224 0.515E+01 0.836E+01
DAV: 11 -0.177780928289E+03 0.37221E+02 -0.85710E+01 9056 0.370E+01 0.771E+01
DAV: 12 -0.184860952607E+03 -0.70800E+01 -0.89813E+00 8712 0.109E+01BRMIX: very serious problems
the old and the new charge density differ
old charge density: 120.00091 new 120.02938
0.797E+01
DAV: 13 -0.157103345513E+03 0.27758E+02 -0.67341E+00 10456 0.222E+01BRMIX: very serious problems
the old and the new charge density differ
old charge density: 120.02938 new 120.13056
0.746E+01
DAV: 14 -0.114396408135E+03 0.42707E+02 -0.51936E+01 11248 0.503E+01BRMIX: very serious problems
the old and the new charge density differ
old charge density: 120.13056 new 120.42305
0.377E+01
WARNING: Sub-Space-Matrix is not hermitian in DAV 14
-1.451855513217620E-006
WARNING: Sub-Space-Matrix is not hermitian in DAV 14
-1.451855513217620E-006
WARNING: Sub-Space-Matrix is not hermitian in DAV 14
-1.451855513217620E-006

it seems that the BZ integration cannot keep charge neutrality at the 12th step.
1) Please check whether your k-mesh is dense enough: in your case, you have to choose NxNx1, where N is the number of k-points you needed for the converged k-mesh for bcc bulk iron.
2) Please also reduce the mixing parameters (AMIX, BMIX, AMIX_MAG and BMIX_MAG), to 0.2 and 0.001 for AMIX and BMIX,... respectively.