# Frequency-dependent susceptibility matrix and related topics

### This page gives hints on how to compute the frequency-dependent susceptibility matrix, and related screened interaction matrix, and inverse dielectric marix with the ABINIT package.

##### Copyright (C) 2016-2017 ABINIT group (MT)

Mentioned in topic_GW, help_features#7.

### Table of content:

- 1. Introduction.
- 2. Related input variables.

### 1. Introduction.

In the independent-particle approximation, the frequency-dependent susceptibility matrix, and related screened interaction matrix, and inverse dielectric matrix can be computed.

This is a prerequisite to many-body perturbation theory calculations, see topic_GW and topic_BSE, to which we refer.

The frequency meshes, used e.g. for integration along the real and imaginary axes, on which the susceptibility matrices (and related matrices) have to be computed are described in topic_FrequencyMeshMBPT.
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### 2. Related input variables.

Compulsory input variables:

... ecuteps [Energy CUT-off for EPSilon (the dielectric matrix)]
... ecutwfn [Energy CUT-off for WaveFunctioNs]
... optdriver [OPTions for the DRIVER]

Basic input variables:

... gwpara [GW PARAllelization level]
... icutcoul [Integer that governs the CUT-off for COULomb interaction]
... nomegasf [Number of OMEGA to evaluate the Spectral Function]
... rcut [Radius of the CUT-off for coulomb interaction]
... vcutgeo [V (potential) CUT-off GEOmetry]

Useful input variables:

... awtr [evaluate the Adler-Wiser expression of $\chi^{0}_{KS}$ assuming Time-Reversal]
... getqps [GET QuasiParticle Structure]
... gwencomp [GW ENergy for COMPleteness]
... irdqps [Integer that governs the ReaDing of QuasiParticle Structure]
... mbpt_sciss [Many Body Perturbation Theory SCISSor operator]
... nbandkss [Number of BANDs in the KSS file]
... nqptdm [Number of Q-PoinTs for the Dielectric Matrix]
... qptdm [Q-PoinTs for the Dielectric Matrix]
... spbroad [SPectral BROADening]
... spmeth [SPectral METHod]

Relevant internal variables:

... %npweps [Number of PlaneWaves for EPSilon (the dielectric matrix)]
... %npwwfn [Number of PlaneWaves for WaveFunctioNs]

Input variables for experts:

... fftgw [FFT for GW calculation]
... gw_invalid_freq [GW treatment of INVALID FREQuency for Hybertsen-Louie PPM]
... gw_nqlwl [GW, Number of Q-points for the Long Wave-Length Limit]
... gw_qlwl [GW, Q-points for the Long Wave-Length limit]
... gwgamma [GW GAMMA]
... gwmem [GW MEMory]
... inclvkb [INCLude VKB]
... kssform [Kohn Sham Structure file FORMat]
... npwkss [Number of PlaneWaves in the KSS file]
... pawoptosc [PAW - OPTion for the computation of the OSCillator matrix elements]
... symchi [SYMmetryze \chi_o]
... zcut [Z-CUT]
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