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General DiscussionDear all,

Is it possible to get eps_2 (imaginary part of the dielectric function) in real units

(for comparison to experiment) instead of arbitrary units ?

Bests

LUCA

Dear all,

Is it possible to get eps_2 (imaginary part of the dielectric function) in real units

(for comparison to experiment) instead of arbitrary units ?

Bests

LUCA

## Hello Luca, Permalink

Submitted by babarker on Mon, 11/21/2016 - 14:35

Hello Luca,

The units of the dielectric function are appropriate. For instance, the macroscopic electronic dielectric constant for GaAs is about 11, from Madelung's Semiconductor: Data Handbook. The result from BerkeleyGW will be a bit larger, around 20 (computed from 1/epsilon^-1(q --> 0, G = G' = 0), since the dielectric function is constructed using the DFT energies. Since these energies underestimate the band gap, the screening is overestimated, and the dielectric function will be larger than the actual system.

If instead you wish to convert the frequency-dependent imaginary epsilon from the BSE code (including using electron-hole interaction or not), you will have to write a script to post-process that data.

Best,

Bradford A. Barker

## Dimensionless Permalink

Submitted by jornada on Thu, 12/01/2016 - 00:07

Hi Luca,

In addition to Brad's answer, remember that eps_2 is dimensionless, so the number reported by BerkeleyGW is the absolute quantity that can be compared against experiment. The only corner case you might experience is when you perform calculations on systems with reduced dimensionality, since you are actually compute the response function on a supercell environment. On such as case, eps_1 and eps_2 are not well-defined quantities anyways for, say, an atomically thin material.

Cheers,

Felipe