ENCUTGW energy cutoff for response function

 ENCUTGW  = real number (energy cutoff for response function)

default: ENCUTGW = ENCUT

The parameter ENCUTGW controls the basis set for the response functions in exactly the same manner as ENCUT does for the wave functions. In the GW case, updates of the response function dominate the computational work load:

$$\frac{1}{\Omega} \sum_{n,n',\mathbf{k}}2 w_{\mathbf{k}} (f... ...hbf{k}+\mathbf{q}}-\epsilon_{n\mathbf{k}} - \omega - i \eta }-$$ (41)

The ENCUTGW controls how many $\mathbf{G}$ vectors are included in the the response function $\chi_{\mathbf{q}}^0 (\mathbf{G}, \mathbf{G}', \omega)$.

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 $G_{\rm cut}$ (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 $\le$ ENCUT, and might crash if ENCUTGW $\ge$ ENCUT). Furthermore, in most cases, it is even possible to set ${\tt ENCUTGW}$ 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 ENCUTFOCK (Sec. 6.64.4), determines the FFT grid in all Hartree-Fock and GW routines. It therefore, influences the behavior and performance of the GW routines (see Sec. 6.64.4). But because FFT's do not dominating the computational work load for GW calculations, savings are small if ENCUTFOCK is set. On the other hand, setting ENCUTFOCK=0 hardly influences the QP-shifts, it does not do any harm to set ENCUTFOCK=0 routinely in GW calculations.