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`ALGO` for response functions and GW calculations

`ALGO`= `CHI | GW0 | GW | scGW | scGW0 | QPGW | QPGW0 `

Default: none.

`ALGO`=CHI calculates the frequency dependent response functions in
the independent particle approximation and in the RPA (or DFT).
The computational effort is fairly small
for bulk systems, but possibly huge for large supercells
with significant vacuum. Usually one wants to determine the response
function at the point only (optically allowed transitions with momentum
transfer ), and this can be achieved by setting
`NKRED` to the number of k-points
in all three direction. For a
k-point grid,
`NKRED=4` will restrict the calculation of the frequency dependent response function to
the important transitions
with momentum transfer
as measured by optical experiments.
For a
grid, one needs to set
`NKREDX=4`, `NKREDY=6` and `NKREDZ=8` (compare Sec. 6.73.9).
Furthermore, we note that `ENCUTGW` can be usually set to fairly small values (see Sec. 6.73.7).
Often one third or one quarter of `ENCUT` (or less) will suffice. Note that
the independent particle response function is independent of `ENCUTGW`,
and the RPA converges reasonably fast with `ENCUTGW`.
Reducing `ENCUTGW`, reduces the storage
requirements and compute time significantly compared to the defaults.

For `ALGO`=GW and `ALGO`=GW0 the orbitals of
the previous groundstate calculations are maintained,
and single shot GW calculations are performed.
If `NELM` is set as well, several iterations are performed,
and the eigenvalues are updated in the calculation of G (`ALGO`=GW0)
or W and G (`ALGO`=GW).
A full update of the orbitals can be performed by specifying
`ALGO`=SCGW and `ALGO`=SCGW0 (QPGW and QPGW0
are synonymous to these setting, and available in VASP.5.2.13).
In the former case, the orbitals and eigenvalues are updated
in G and W, whereas in the latter case the
orbitals and eigenvalues are only updated in G.
Convergence of the eingevalues with respect to `ENCUTGW` (see Sec. 6.73.7 and the
number of unoccupied bands `NBANDS` is usually
fairly slow and should be checked carefully.

We strongly recommend to read the following literature before performing calculations using VASP
[111,112,113,114].

N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at