Using the GW routines for the determination of frequency dependent dielectric matrix

The GW routine also determines the frequency dependent dielectric matrix
without local field effects and with local field effects in the random
phase approximation (RPA, `LRPA`=.TRUE.),
or the DFT approximation (`LRPA`=.FALSE, see Sec. 6.72.5).
The calculated microscopic frequency dependent dielectric function,
must match exactly those determined using the optical
routine (`LOPTICS`=.TRUE. see Sec. 6.72.1), and, in the static limit,
the density functional perturbation routines (`LEPSILON`=.TRUE. see Sec. 6.72.4).
In fact, it is guaranteed that the results are identical to those determined
using a summation over conduction band states (Sec. 6.72.1).
Differences for `LSPECTRAL`=.FALSE. must be negligible,
and can be solely related to a different complex shift `CSHIFT`
(defaults for `CSHIFT` are different in both routines).
Setting `CSHIFT` manually in the INCAR file will remedy this issue.
If differences prevail, it might be required to increase `NEDOS`
(in this case the `LOPTICS` routine was suffering from an
inaccurate frequency sampling, and the GW routine was most likely
performing perfectly well). For `LSPECTRAL`=.TRUE.
differences can arise, because (i) the GW routine uses
less frequency points and different frequency grids than the
optics routine or again (ii) from a different complex shift.
Increasing `NOMEGA` should remove all discrepancies.
Finally, the GW routine is the only routine capable to include
local field effects for the frequency dependent dielectric function.

The imaginary and real part of frequency dependent dielectric function is always determined by the GW routine. It can be conveniently grepped from the file using the command (note two blanks between the two words)

grep " dielectric constant" OUTCARThe first value is the frequency (in eV) and the other two are the real and imaginary part of the trace of the dielectric matrix. Note that two sets can be found on the OUTCAR file. The first one corresponds to the head of the microscopic dielectric matrix (and therefore does not include local field effects), whereas the second one is the

If full GW calculations are not required, it is possible to greatly accelerate the calculations, by calculating the response functions only at the -point. This can be achieved by setting (see Sec. 6.73.9):

NKREDX = number of k-points in direction of first lattice vector NKREDY = number of k-points in direction of second lattice vector NKREDZ = number of k-points in direction of third lattice vectorThe calculation of the QP shifts can be bypassed by setting