non-conservation of electrons

[ashwin]

Dear VASPers,

I am running into a strange situation where some k-points in my OUTCAR file are missing electrons. My system is a SiC slab terminated with hydrogens on one surface and graphitized on the other surface. There are 183 electrons in all---of these, 182 are *always* in the valence band well below the Fermi level while the 183rd electron is (mostly) in a spin-up level (band 92) very close to the Fermi level. The conduction bands are well above the Fermi level, so all the problems arise from the state at the Fermi level.

When I look at the occupation of band 92 at each k-point (see below), I see that in some cases the energies of the spin-up and spin-down bands go above the Fermi level and the occupation goes to zero (as it should). My problem is that I don't see the electron in any other band, which I take to mean that the electron has mysteriously disappeared (182 instead of 183 electrons). In other instances, the energy of both the spin-up and spin-down bands drop below the Fermi level and both contain an electron, which seems to suggest that an additional electron has appeared (184 instead of 183 electrons).

I am using PAW-PBE for Si, C and H. I have tried increasing my energy cutoff up to 500 eV and my k-point sampling to 12x12x1 (Gamma centered for hexagonal supercell), which are both well above what I require for total energy convergence (300 eV and 5x5x1, respectively). Switching from Bloechl tetrahedron method (ISMEAR=-5, SIGMA=0.01) to Fermi smearing has no effect either.

Any suggestions would be gratefully accepted!

-Ashwin.

--- begin INCAR ---

SYSTEM = SiCGr
ISTART = 0
ENCUT = 500
ISMEAR = -5
SIGMA = 0.01
LORBIT = 11
IALGO = 48
ISPIN = 2
LWAVE = .FALSE.
# use recommended NGX, NGY, NZG from OUTCAR
NGX = 42
NGY = 40
NGZ = 218
EDIFF = 1.e-5
ISYM = 0
SYMPREC = 1.e-10

--- end ---


--- occupation of band 92 ---

E-fermi : 4.4784 XC(G=0): -7.6887 alpha+bet : -9.9495

BZINTS: Fermi energy: 4.478373;183.000000 electrons
Band energy:-596.826371; BLOECHL correction: -0.011004

Spin up Spin down
Energy Occ. Energy Occ.
4.1573 1.00000 4.5388 0.00000
4.2156 1.00000 4.5694 0.00000
4.3472 1.01463 4.6672 0.00000
4.4977 0.57864 4.8029 0.00000
4.6268 -0.15183 4.9257 0.00000
4.7078 0.00000 5.0003 0.00000
4.7332 0.00000 4.8600 0.00000
4.2156 1.00000 4.5694 0.00000
4.2156 1.00000 4.5694 0.00000
4.2906 1.00000 4.6104 0.00000
4.3680 1.02652 4.6585 0.00000
4.4434 0.80795 4.7234 0.00000
4.5066 0.37700 4.7845 -0.04961
4.5383 0.27363 4.8138 -0.07464
4.5335 0.28440 4.8074 -0.07394
4.4972 0.41825 4.7718 -0.05858
4.4373 0.84152 4.7147 0.00000
4.3671 1.02407 4.6570 0.00000
4.2906 1.00000 4.6104 0.00000
4.3472 1.01463 4.6672 0.00000
4.2906 1.00000 4.6104 0.00000
4.3472 1.01463 4.6672 0.00000
4.3671 1.02407 4.6570 0.00000
4.3007 1.00000 4.5696 0.00000
4.2275 1.03368 4.4943 0.25965
4.1772 1.07246 4.4481 0.63593
4.1563 1.07101 4.4288 0.69814
4.1712 1.07378 4.4410 0.66333
4.2228 1.02406 4.4887 0.28721
4.3007 1.00000 4.5696 0.00000
4.3680 1.02652 4.6585 0.00000
4.4977 0.57864 4.8029 0.00000
4.3671 1.02407 4.6570 0.00000
4.3680 1.02652 4.6585 0.00000
4.4977 0.57864 4.8029 0.00000
4.4373 0.84152 4.7147 0.00000
4.2228 1.02406 4.4887 0.28721
3.9830 1.00000 4.2593 1.10941
3.8057 1.00000 4.1014 1.00000
3.8043 1.00000 4.1000 1.00000
3.9830 1.00000 4.2593 1.10941
4.2275 1.03368 4.4943 0.25965
4.4434 0.80795 4.7234 0.00000
4.6268 -0.15183 4.9257 0.00000
4.4373 0.84152 4.7147 0.00000
4.3007 1.00000 4.5696 0.00000
4.4434 0.80794 4.7234 0.00000
4.6268 -0.15183 4.9257 0.00000
4.4972 0.41825 4.7718 -0.05858
4.1712 1.07378 4.4410 0.66333
3.8043 1.00000 4.1000 1.00000
3.5851 1.00000 3.9228 1.00000
3.8057 1.00000 4.1014 1.00000
4.1772 1.07246 4.4481 0.63593
4.5066 0.37700 4.7845 -0.04961
4.7078 0.00000 5.0003 0.00000
4.4972 0.41825 4.7718 -0.05858
4.2228 1.02406 4.4887 0.28721
4.2275 1.03368 4.4943 0.25965
4.5066 0.37699 4.7845 -0.04961
4.7078 0.00000 5.0003 0.00000
4.5335 0.28440 4.8074 -0.07394
4.1563 1.07101 4.4288 0.69814
3.8057 1.00000 4.1014 1.00000
3.8043 1.00000 4.1000 1.00000
4.1563 1.07101 4.4288 0.69814
4.5383 0.27363 4.8138 -0.07464

--- end ---

[admin]

please carefully sum up the occupancies of these 2 bands for up and down states (including the proper k-point weights) to check.
If charge neutrality really would have been violated, your would have obtained a warning from vasp.