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`ENCUTFOCK`: FFT grid in the Hartree-Fock related routines

`ENCUTFOCK`= [real]

Default: none

The flag `ENCUTFOCK` is no longer supported in VASP.5.2.4 and newer versions.
Please use `PRECFOCK` instead (see Sec. 6.71.5).

The `ENCUTFOCK` tag sets the energy cutoff that determines the FFT grids used by the Hartree-Fock routines.
The only sensible value for `ENCUTFOCK` is `ENCUTFOCK`=0.
This implies that the smallest possible FFT grid, which just encloses the cutoff sphere
corresponding to the plane wave cutoff, 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 FFT grid used internally in the exact exchange (Hartree-Fock) routines
is written to the OUTCAR file. Simply search for lines starting with

FFT grid for exact exchange (Hartree Fock)

In many cases, a sensible approach is to determine the electronic and ionic groundstate
using `ENCUTFOCK`=0, and to make one final total energy calculation
without the flag `ENCUTFOCK`.

N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at