ENCUTGW= [real] (energy cutoff for response function)
The parameter ENCUTGW controls the basis set for the response functions in exactly the same manner as ENCUT does for the orbitals. In the GW case, updates of the response function dominate the computational work load:
Tests have shown that choosing ENCUTGW=ENCUT yields essentially exact results. In principle, however, the response function contains contributions up to twice the plane wave cutoff (see Sec. 7.2). Since the diagonal of the dielectric matrix converges rapidly to one, such a large cutoff is never actually required (the present release has only been tested for ENCUTGW ENCUT, and might crash if ENCUTGW ENCUT). Furthermore, in most cases, it is even possible to set ENCUTGW to a value between 150 to 200 eV, and even 100 eV gives usually QP shifts that are accurate to within a few hundreds of an eV (0.01-0.02 eV). This can help to speed up the calculations significantly and reduces the memory requirements substantially.
The flag PRECFOCK (Sec. 6.71.5), determines the FFT grid in all GW (and Hartree-Fock) related routines. For small systems (which are often dominated by FFT operations), it 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.