`PRECFOCK`= `Low | Medium | Fast | Normal | Accurate`

Default: `PRECFOCK`=Normal

The `PRECFOCK` parameter controls the FFT grid for the exact exchange (Hartree-Fock) routines,
i.e. it is possible to chose a different grid for the exact exchange part, and for
the local Hartree and DFT potentials. In fact, the exchange is rather insensitive
to the FFT grids, and in many cases a rather coarse grid can be used to calculate
the overlap density and the potentials. Since the exact exchange requires
the evaluation of an overlap density (compare 6.57)

For `Low` and `Fast`, however, the smallest possible FFT grid, which just encloses the cutoff sphere
(
) determined by the plane wave cutoff (`ENCUT`), is used.
This accelerates the calculations by roughly a factor two to three, but
causes slight changes in the total energies and some noise in the
calculated forces. The corresponding FFT grid that is used in the Hartree Fock routines
is written to the OUTCAR file after the lines

FFT grid for exact exchange (Hartree Fock)For

For `PRECFOCK`=`Fast`, `Normal` and `Accurate`,
the augmentation charges--which are required to restore the norm and dipoles of
the overlap density on the plane wave grid --are made soft,
such that an accurate presentation on the plane wave grid is possible
even for relatively coarse FFT grids. The sphere size is printed out
after

Radii for the augmentation spheres in the non-local exchangeThe following table summarises the possible setting:

PRECFOCK |
FFT grid | augmentation charge | advantage/disatvantage |

VASP.5.2.2 compatible, not recommended |
|||

Low |
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 |

VASP.5.2.4 and newer, recommended | |||

Fast |
very soft augmentation charge | some noise in forces/good energy | |

Normal |
3/2 | soft augmentation charge | accurate forces and energy |

Accurate |
2 | soft augmentation charge | very accurate forces and energy |

soft augmentation charge: radius for augmentation sphere is increased by factor 1.25 compared to default

very soft augmentation charge: radius is increased by factor 1.35 compared to default except for like charge, for the channel the radius of the augmentation sphere is increased by a factor 1.25

Even `PRECFOCK`=Fast yields fairly low noise in the forces
and virtually no egg-box effects (aliasing errors). In the forces, the noise is usually
below 0.01 eV/Å. For `PRECFOCK`=N and `PRECFOCK`=A, noise is usually
not an issue, and the accuracy is sufficient even for phonon calculations in large supercells.