Im(eps) not zero for energies < band gap

Problems running VASP, runtime errors, technical questions.
Please check whether the answer to your question is given in
the VASP online manual or has been discussed in this forum
previously!

Moderators: Global Moderator, Moderator

Im(eps) not zero for energies < band gap

Postby rsaniz » Fri Dec 22, 2017 4:57 pm

Dear VASP support team,

Analyzing the result of the calculation of the dielectric function of a semiconductor (LOPTICS = .True.), I found that the imaginary part [Im(eps)] in not zero even at energies well below the band gap value. In fact, the imaginary part departs from zero as soon as the energy is > 0. This is, of course, unphysical, and can lead to erroneous results for quantities depending on the dielectric function. I found the inaccuracy both with PBE and HSE06.

The problem greatly reduced if one uses a CSHIFT much smaller than the default one, e.g., CSHIFT=0.01, instead of CSHIFT=0.1. I ran a test using a 21x21x21 k-point mesh using VASP version 5.4.4 (PBE calculation), comparing the result with both CSHIFTs. But the imaginary part of the dielectric function for CSHIFT=0.01, although small, is still not zero at energies > 0 below the band gap value. Also, the curve for CSHIFT=0.01 is not ideal, as it is jagged. In principle a higher density k-point mesh should solve this. However, performing a HSE06 calculation over such dense k-point meshes becomes computationally prohibitive, specially for systems such as the present one. This is unfortunate, because for a semiconductor one would like to use HSE06. I wonder how the calculations can be improved having hybrid functionals in mind.

The SCF calculation was done with the following INCAR and KPOINTS files:
INCAR
SYSTEM = Pb2SnSbO6.5
ENCUT = 450
PREC = Normal
ISMEAR = 0
SIGMA = 0.01
LREAL = Auto
EDIFF = 1.E-05
NSIM = 4
NBANDS = 120
NPAR = 120
KPAR = 2
LMAXMIX = 4

KPOINTS
Pb2SnSbO6.5
0 ! automatically generate k-points
Gamma !To generate k points via MP method
4 4 4 !the number of divisions along basic vector
0. 0. 0. !the shift

The dielectric function calculation was done with the following INCAR and KPOINTS files:
INCAR
SYSTEM = Pb2SnSbO6.5
ENCUT = 450
PREC = Normal
ISMEAR = 0
SIGMA = 0.01
LREAL = Auto
EDIFF = 1.E-08
NSIM = 4
NBANDS = 240
NPAR = 120
KPAR = 2
LMAXMIX = 4
LOPTICS = .TRUE.
CSHIFT = 0.01
NEDOS = 10000

KPOINTS
Pb2SnSbO6.5
0 ! automatically generate k-points
Gamma !To generate k points via MP method
21 21 21 !the number of divisions along basic vector
0. 0. 0. !the shift

The POTCARs used for the metals included the d electrons.

Best regards,
Rolando Saniz
rsaniz
Newbie
Newbie
 
Posts: 1
Joined: Thu May 10, 2012 2:15 pm

Return to Bugreports

Who is online

Users browsing this forum: Google [Bot] and 1 guest

cron