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# ENCUTGW

**ENCUTGW** = [real]

Default: **ENCUTGW** = 2/3 ENCUT

Description: The tag **ENCUTGW** sets the energy cutoff for response function. It controls the basis set for the response functions
in exactly the same manner as ENCUT does for the orbitals.

In GW and RPA calculations, storing and manipulating the response function dominates the computational work load:

**ENCUTGW** controls how many vectors are included in the
the response function
.

Our experience suggests that choosing **ENCUTGW**= 2/3 ENCUT yields
reasonable results at fairly modest computational cost, although, the response function
contains contributions up to twice the plane wave cutoff
(see
ALGO).
Furthermore, RPA correlation energies are reported using an internal extrapolation of the correlation
energy by varying **ENCUTGW** internally (inside VASP) between the largest value given in the INCAR
file and smaller values ^{[1]}. The extrapolated value is only reliable, if
**ENCUTGW** is smaller then ENCUT (the cutoff extrapolation with respect to **ENCUTGW**
would be very precise, if the plane wave basis for the orbitals were infinite).
Again the VASP defaults yield very reasonable values for the extrapolated correlation
energy. In fact, it is unwise to increase **ENCUTGW** only, without increasing ENCUT .
To converge RPA correlation energies, simply increase ENCUT and the number
of orbitals, and use the VASP default for **ENCUTGW**.

For QP gaps, it is sometimes possible to set **ENCUTGW**
to values between 150 to 200 eV, and even 100 eV can yield
gaps that are accurate to within a few tens of an eV for main group elements.
Be aware, however, that the absolute values of the QP energies
depend inverse proportionally on the number of plane waves. Thus
convergence of absolute QP energies is always very slow, although QP gaps
might seem converged.

The recommended procedure to obtain accurate QP energies is discussed in the reference below. Specifically, for reference type calculations we recommend the following procedure:

- use the default for ENCUTGW, or even decrease ENCUTGW to half the value of ENCUT.
- Calculate all orbitals that the plane wave basis set allows to calculate. This number can be determined by searching for "maximum number of plane-waves" in the groundstate DFT OUTCAR file, and setting NBANDS to this value.
- Increase ENCUT systematically and plot the QP energies versus the number of plane wave coefficients (which equals the number of orbitals). This means ENCUTGW and NBANDS increase as ENCUT increases.

This procedure can be carried out using few k-points. Other commonly applied methods can yield less accurate results and are not considered to be reliable.

## FFT grid and PRECFOCK

The flag PRECFOCK determines the FFT grid in all GW (and Hartree-Fock) related routines.
For small systems (which are often dominated by FFT operations),
the flag can have a significant impact on the compute time for
QP calculations. For large systems, the FFT's usually do not
dominating the computational work load and savings are expected to be small for PRECFOCK = *fast*.
QP shifts are usually not very sensitive to the setting of PRECFOCK
(and it therefore does not harm to set PRECFOCK = *fast*), whereas for
RPA calculations we recommend to set PRECFOCK = *normal* to avoid numerical errors.

## Related Tags and Sections

PRECFOCK, ENCUT, ENCUTGWSOFT, GW calculations, ACFDT/RPA calculations

## Further reading

- A comprehensive study of the performance of the convergence of GW calculations can be found
^{[2]}. Generally QP energies converge like one over the number of orbitals and one over the number of plane waves in the response function. For basis set converged calculations we recommend to use the strategies in Ref.^{[2]}

## References

- ↑ [https://doi.org/10.1103/PhysRevB.77.045136 J. Harl, and G. Kresse, "Cohesive energy curves for noble gas solids calculated by adiabatic connection fluctuation-dissipation theory.", Phys. Rev. B 77.4 045136 (2008).]
- ↑
^{a}^{b}J. Klimes, M. Kaltak, and G. Kresse, Phys. Rev. B 90, 075125 (2014)