# Epsilon code overview

Epsilon is the name of the code that generates the polarizability matrix and the inverse dielectric matrix for a bulk or nanoscale system. The main result of the epsilon code is the generation of the epsmat[.h5] and eps0mat[.h5] files which can be used in a Sigma or BSE calculation.

## Summary of input and output files

### Required input files

• epsilon.inp: Input parameters.

• WFN: This is linked to the unshifted grid. Information of spinor wavefunctions is initialized automatically.

• WFNq: This is linked to the shifted grid. When a shift is not required, this can be the same file as WFN. This happens for calculations on:

• semiconductors with spherical or box truncation;
• metals / graphene-like screening with the is_q0 flag set to 2.

### Auxiliary input files

These files are output files from previous runs, and may be used as input to speed up calculation:

• chimat[.h5]: Polarizability matrix for $q\ne0$. No need to recalculate matrix elements. The file has a .h5 extension if the code is compiled with HDF5 support (for specification, see epsmat.h5.spec).

• chi0mat[.h5]: Polarizability matrix for $q\rightarrow0$. No need to recalculate matrix elements. The file has a .h5 extension if the code is compiled with HDF5 support (for specification, see epsmat.h5.spec).

The files below are used if eqp_corrections is set in epsilon.inp. The corrected eigenvalues are used for constructing the polarizability matrix.

• eqp.dat: A list of quasiparticle energy corrections for the bands in WFN.
• eqp_q.dat: A list of quasiparticle energy corrections for the bands in WFNq.

### Output Files

• espmat[.h5]: Inverse dielectric matrix for $q\ne0$. The file has a .h5 extension if the code is compiled with HDF5 support (for specification, see epsmat.h5.spec).

• eps0mat[.h5]: Inverse dielectric matrix for $q\rightarrow0$. The file has a .h5 extension if the code is compiled with HDF5 support (for specification, see epsmat.h5.spec).

• epsilon.log: The log file containing values of chimat and epsmat.

• chi_converge.dat: Convergence chi with respect to empty orbitals. Columns:

• number of conduction bands;

• $\mathrm{Re}\, \chi(G=G'=0,q)$, extrapolated $\mathrm{Re}\,\chi(G=G'=0,q)$;

• $\mathrm{Re}\, \chi(G=G'=G_\mathrm{max},q)$, extrapolated $\mathrm{Re}\,\chi(G=G'=G_\mathrm{max},q)$.