PRECFOCK
PRECFOCK = Low | Medium | Fast | Normal | Accurate
Default: PRECFOCK = Normal
Description: PRECFOCK controls the FFT grids used in the exact exchange routines (Hartree-Fock and hybrid functionals).
The FFT grid used for the exact exchange contributions may differ from the one used for the local Hartree and DFT potentials. In fact, the exact exchange is rather insensitive to the FFT grids, and in many cases a rather coarse grid can be used to calculate the overlap densities and the potentials. Since the exact exchange requires the evaluation of an overlap density:
- [math]\displaystyle{ \phi_{\mathbf{k}n}^{*}(\mathbf{r})\phi_{\mathbf{q}m}(\mathbf{r}) }[/math]
errors in the convolution (aliasing errors) are only avoided, if the FFT grid contains all Fourier components up to twice the plane wave with the largest wave vector, 2|Gcut|.
PRECFOCK FFT-grid Augmentation charge Quality Low Gcut identical to standard DFT large noise in forces/energy errors Medium identical to std. FFT identical to standard DFT some noise in forces/good energy Fast Gcut very soft augmentation chargea some noise in forces/good energy Normal 3/2×Gcut soft augmentation chargeb accurate forces and energy Accurate 2×Gcut soft augmentation chargeb very accurate forces and energy a radius for augmentation sphere is increased by factor 1.25 compared to default. b radius is increased by factor 1.35 compared to default except for s -like charge, for the s -channel the radius of the augmentation sphere is increased by a factor 1.25.
where
- [math]\displaystyle{ E_{\rm cut}=\frac{\hbar^2}{2m_e}G_{\rm cut}^2 }[/math]
with Ecut=ENCUT.
Related Tags and Sections
ENCUT, hybrid functionals, settings for specific hybrid functionals