GGA_COMPAT
GGA_COMPAT = .TRUE. | .FALSE.
Default: GGA_COMPAT = .TRUE.
Description: If set to GGA_COMPAT = .FALSE., this flag restores the full lattice symmetry for gradient corrected functionals.
GGA and METAGGA functionals might break the symmetry of the Bravais lattice slightly for non cubic cells (this includes primitive fcc and bcc lattices). The origin of this problem is subtle and relates to the fact that the gradient field breaks the lattice symmetry for non-cubic lattices. This can be fixed by setting
GGA_COMPAT = .FALSE.
to apply a spherical cutoff to the gradient field, i.e. for all reciprocal lattice vectors [math]\displaystyle{ \bold{G} }[/math] that exceed a certain cutoff length [math]\displaystyle{ \bold{G}_{cut} }[/math] the gradient field as well as the charge density are set to zero before calculating the exchange correlation energy and potential. The cutoff [math]\displaystyle{ \bold{G}_{cut} }[/math] is determined automatically so that the cutoff sphere is fully inscribed in the parallelepiped defined by the FFT grid in the reciprocal space.
Mind: For compatibility reasons with older versions of VASP, the default is GGA_COMPAT=.TRUE. However, setting the flag usually changes the energy only in the sub meV energy range (0.1 meV), and for most results it does matter little how GGA_COMPAT is set. The most important exception is for the calculation of magnetic anisotropy, for which we strongly recommend GGA_COMPAT=.FALSE.