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CSHIFT = [real]
|Default: CSHIFT||= OMEGAMAX*1.3 / max(NOMEGA,40)||for GW calculations|
|= 0.1||for linear response calculations|
Description: CSHIFT sets the (small) complex shift η in the Kramers-Kronig transformation.
The default CSHIFT=0.1 is perfectly acceptable for most calculations and causes a slight smoothening of the real part of the dielectric function. If the gap is very small (i.e. approaching two times CSHIFT), slight inaccuracies in the static dielectric constant are possible, which can be remedied by decreasing CSHIFT. If CSHIFT is further decreased, it is strongly recommended to increase the parameter NEDOS to values around 2000.
For GW type calculations, one should manually check that CSHIFT is at least as large as the grid spacing at low frequencies. If CSHIFT is smaller than the grid spacing, the QP energies might show erratic behavior (for instance large re-normalization factors Z)