PRECFOCK= Low | Medium | Fast | Normal | Accurate
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 PRECFOCK=Normal, the FFT grid for the exact exchange is identical to the FFT grid used for the orbitals for PREC=Normal in the DFT part. For PRECFOCK=Accurate, the FFT grid for the exact exchange is identical to the FFT grid used for the orbitals for PREC=Accurate in the DFT part (any combination of PREC and PRECFOCK is allowed).
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|
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.