Optical proprieties with Quantum espresso

Question

I am a beginner in Quantum Espresso, and I want to calculate optical properties. When I calculate the dielectric functions, it gives me four output files. I don't know which files correspond to the real function and imaginar the dielectric function.

Above are the input and output files Input file:

&INPUTPP
calculation     = 'eps',
outdir          = './outdir' 
/

&ENERGY_GRID
 smeartype      = 'gauss',
 intersmear     = 0.15d0,
 intrasmear     = 0.0d0,
 wmax           = 10.0d0,
 wmin           = 0.0d0,
 nw             = 600,
 shift          = 0.0d0,

output files:

eels_CNT(10.0)@C6H2.dat
epsi_CNT(10.0)@C6H2.dat
epsr_CNT(10.0)@C6H2.dat
ieps_CNT(10.0)@C6H2.dat

Answer 1

There will be 4 output files in total for an epsilon.x run. These are:

eels_prefix.dat
epsi_prefix.dat
epsr_prefix.dat
ieps_prefix.dat

where prefix is the prefix of your calculation.

The epsr_ and epsi_ files contain the real $Re(ε(ω))$ and imaginary $Im(ε(ω))$ parts of the diagonal part of the dielectric tensor, respectively. The eels_ file contains the electron energy loss spectra, $Im(1/ε(ω))$ calculated from the diagonal elements of the dielectric tensor. The ieps_ file contains the diagonal components of the dielectric tensor, $ε(iω)$, calculated on the imaginary frequency axis.

If you open the epsr_ file, you will get 4 columns: energy $E \,(eV)$, and the real parts of the dielectric function, $Re(ε_{xx}(ω)),\; Re(ε_{yy}(ω)),$ and $Re(ε_{zz}(ω))$ in the $x‐, \;y‐,$ and $z‐$ directions, respectively. The same goes for epsi_ in case of imaginary parts.

Reference: Tuan Hung, N., Nugraha, A.R.T., & Saito, R. (2022). Quantum ESPRESSO Course for Solid-State Physics (1st ed.). Jenny Stanford Publishing. https://doi.org/10.1201/9781003290964. pp: 118-119.