Numerical error in Fermi level - whether system should be classified as insulator or metal
Posted: Tue Apr 05, 2022 1:53 am
Hi all,
I have a question about the methodology for deciding whether my system would a metal or insulator. The material doesn't have experimental data, so there is no real-life comparison unfortunately.
My system has a gap between the valence and conduction bands (or I should say between the two bands closest to the estimated Fermi level value on either side).
I read from the numerous posts previously in this forum that the Fermi level for gapped systems can be placed by VASP anywhere from the bottom of conduction band to the top of valence bands.
What I am interested in, is the case where the Fermi level at the end of OUTCAR is determined to be close to the top of VB/bottom of CB but differ very closely to the Kohn-Sham energies list in the OUTCAR file.
For example, at the end of OUTCAR file the fermi energy is printed as:
BZINTS: Fermi energy: 3.103416; 200.000000 electrons
Band energy:-0.123909E+04; BLOECHL correction: -0.000000
while the K-S energies around this value has occupancy very close to 1 but not exactly 1.
k-point 209 : 0.5000 0.0000 0.3750
band No. band energies occupation
....
98 1.9169 1.00000
99 3.1034 0.99988
100 3.1034 0.99978
101 4.4895 0.00000
102 4.4895 0.00000
....
Similarly, there are also other cases where the Fermi energy is placed near the bottom of conduction band but has occupancy very close but not equal to 0: e. g. 0.00015.
In both cases, there is a clear gap between the VB and CB.
I know that ideally insulators' occupancy should be a step function going abruptly from 1 to 0 across fermi level, but because even for insulating systems with confirmed experimental data sometimes VASP also gives this minute deviation from zero/unity occupation, I think my material is an insulator and this small deviation could be numerical error-related.
Since I do not know exactly whether my material is metal or insulator in reality (no experiments yet), are there things that I can do more to further convince myself that my system is an insulator or a metal?
I tried using denser k-meshes, small sigma (<= 0.01) but this doesn't always eliminate these occurrences.
May I know if you can share a more systematic approach to deal with determining whether an unknown material is insulator/metal?
thank you.
I have a question about the methodology for deciding whether my system would a metal or insulator. The material doesn't have experimental data, so there is no real-life comparison unfortunately.
My system has a gap between the valence and conduction bands (or I should say between the two bands closest to the estimated Fermi level value on either side).
I read from the numerous posts previously in this forum that the Fermi level for gapped systems can be placed by VASP anywhere from the bottom of conduction band to the top of valence bands.
What I am interested in, is the case where the Fermi level at the end of OUTCAR is determined to be close to the top of VB/bottom of CB but differ very closely to the Kohn-Sham energies list in the OUTCAR file.
For example, at the end of OUTCAR file the fermi energy is printed as:
BZINTS: Fermi energy: 3.103416; 200.000000 electrons
Band energy:-0.123909E+04; BLOECHL correction: -0.000000
while the K-S energies around this value has occupancy very close to 1 but not exactly 1.
k-point 209 : 0.5000 0.0000 0.3750
band No. band energies occupation
....
98 1.9169 1.00000
99 3.1034 0.99988
100 3.1034 0.99978
101 4.4895 0.00000
102 4.4895 0.00000
....
Similarly, there are also other cases where the Fermi energy is placed near the bottom of conduction band but has occupancy very close but not equal to 0: e. g. 0.00015.
In both cases, there is a clear gap between the VB and CB.
I know that ideally insulators' occupancy should be a step function going abruptly from 1 to 0 across fermi level, but because even for insulating systems with confirmed experimental data sometimes VASP also gives this minute deviation from zero/unity occupation, I think my material is an insulator and this small deviation could be numerical error-related.
Since I do not know exactly whether my material is metal or insulator in reality (no experiments yet), are there things that I can do more to further convince myself that my system is an insulator or a metal?
I tried using denser k-meshes, small sigma (<= 0.01) but this doesn't always eliminate these occurrences.
May I know if you can share a more systematic approach to deal with determining whether an unknown material is insulator/metal?
thank you.