For gradient corrected functionals the exchange correlation functional 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. To fix this, a spherical cutoff is applied to the gradient field for GGA_COMPAT = .FALSE., e.g. for all reciprocal lattice vectors that exceed a certain cutoff length the gradient field as well as the charge density is set to zero before calculating the exchange correlation energy and potential. The cutoff is determined automatically so that the cutoff sphere is fully inscribed in the parallelepiped defined by the FFT grid in the reciprocal space.
This flag restores the full lattice symmetry for gradient corrected functionals, and we therefore recommend to set
GGA_COMPAT = .FALSE.for all gradient corrected calculations. For compatibility reasons, the default is GGA_COMPAT = .TRUE. until VASP.5.2. 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 are magnetic anisotropies, for which we strongly recommend to set GGA_COMPAT = .FALSE..