strange energy ordering in G0W0 calculation

[giacomo giorgi]

Dear All,

I am testing GW calculation on VASP.
My system is a cell of 8atoms of GaAs. I made the PAW/LDA calculations in order to get initial eigenfunctions for G0W0 one shot calculation.

I observe an anomalous energy ordering of the bands at gamma point

k-point 1 : 0.0000 0.0000 0.0000
band No. DFT-energies QP-energies sigma(DFT) V_xc(DFT) V^pw_x(r,r') Z occupation

1 -11.9046 -16.3637 -19.3366 -13.3055 -26.1458 0.7393 2.0000
2 -11.9046 -16.3637 -19.3366 -13.3055 -26.1458 0.7393 2.0000
3 -11.9046 -16.3637 -19.3366 -13.3055 -26.1458 0.7393 2.0000
... ... ... ... ...
... ... ... ... ...
... ... ... ... ...
... ... ... ... ...
34 3.1590 3.2678 -11.8885 -12.0196 -12.6925 0.8298 2.0000
35 3.1590 3.2678 -11.8885 -12.0196 -12.6925 0.8298 2.0000
36 3.1590 3.2678 -11.8885 -12.0196 -12.6925 0.8298 2.0000
37 3.6866 5.7994 -9.7011 -12.2223 -7.9087 0.8380 0.0000
38 4.4551 5.5053 -7.6243 -8.8672 -5.0165 0.8449 0.0000


This means that the energy of the conduction band (37th band) is higher than that of the band (38th band) immediately above, i.e., 5.7994 eV vs. 5.5053 eV. It seems indeed that at DFT level the correct ordering is maintained

I do not think this is an usual behaviour. Any opinion about? Where am I failing?
For sake of completeness I attach the INCAR I used. KPOINTS is Gamma centered 6x6x6.

INCAR

System = GW-GaAs


ENCUTGW=90
ISMEAR = 0
SIGMA = 0.05
LOPTICS = .TRUE.
ALGO = GW0
NOMEGA = 64
NBANDS = 128


Thanks in advance,
G

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