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).

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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:

$\phi_{\mathbf{k}n}^{*}(\mathbf{r})\phi_{\mathbf{q}m}(\mathbf{r})$

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|G_cut|.

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

$E_{\rm cut}=\frac{\hbar^2}{2m_e}G_{\rm cut}^2$

with E_cut=ENCUT.

Related Tags and Sections

ENCUT, hybrid functionals, settings for specific hybrid functionals

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