ENCUTGW

ENCUTGW = [real] (energy cutoff for response function
Default: ENCUTGW = ENCUT 

{\tt ENCUTGW}= [real] (energy cutoff for response function)

Default: {\tt ENCUTGW}={\tt ENCUT}

The parameter {\tt ENCUTGW} controls the basis set for the response functions in exactly the same manner as {\tt ENCUT} does for the orbitals. In the GW case, updates of the response function dominate the computational work load: \begin{equation}

   \frac{1}{\Omega} \sum_{n,n',{\mathbf{k}}}2 w_{{\mathbf{k}}}
(f_{n'{\mathbf{k}}+{\mathbf{q}}} - f_{n{\mathbf{k}}})  

\times \frac{\langle \psi_{n{\mathbf{k}}}| e^{-i ({\mathbf{q}}+{\mathbf{G}}){\mathbf{r}}} | \psi_{n'{\mathbf{k}}+{\mathbf{q}}}\rangle \langle \psi_{n'{\mathbf{k}}+{\mathbf{q}}}| e^{i ({\mathbf{q}}+{\mathbf{G}}'){\mathbf{r'}}} | \psi_{n{\mathbf{k}}}\rangle}

{ \epsilon_{n'{\mathbf{k}}+{\mathbf{q}}}-\epsilon_{n{\mathbf{k}}} -  \omega - i \eta }-

\label{equ:chi1} \end{equation} The {\tt 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 {\tt ENCUTGW}={\tt 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. \ref{algo-wrap}). 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 {\tt ENCUTGW} $\le$ {\tt ENCUT}, and might crash if {\tt ENCUTGW} $\ge$ {\tt 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.

\index{INCAR!P!PRECFOCK|textit} The flag {\tt PRECFOCK} (Sec.~\ref{incar-precfock}), 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 {\tt PRECFOCK = fast }. QP shifts are usually not very sensitive to the setting of {\tt PRECFOCK} (and it therefore does not harm to set {\tt PRECFOCK = fast }), whereas for RPA calculations we recommend to set {\tt PRECFOCK= normal} to avoid numerical errors.