[Wien] non-zero value of epsilon 2 below the energy band gap

[Anup Shakya]

Dear All,

I have performed calculations for two double perovskite oxide materials and
the band gap of the material is found to be more than 1. 3 eV for both
materials. The calculations have been performed using GGA+U, since it
contains rare earth materials. The value of U have been used from the
literature. The energy convergence was performed till 0.0001 Ry and the
optical properties were calculated using optic. However, the imaginary part
of the dielectric constant (epsilon 2) shows non-negligible value below the
energy band gap. PBE was used as the exchange correlation functional and if
I am not wrong then for the calculation of epsilon 2 the contribution of
inter-band transitions are taken into consideration and the intra-bands are
neglected. Then what is the reason for the non-zero value of epsilon 2
below the energy band gap?
 If anyone could suggest some views, I would be very grateful. Please
let me know, if anyone needs more information.

Sincerely,
Anup Pradhan Sakhya,
TIFR.
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