GGA_COMPAT

GGA_COMPAT = .TRUE. | .FALSE.
Default: GGA_COMPAT = .TRUE.

Description: If set to GGA_COMPAT = .FALSE., this tag restores the full lattice symmetry for gradient-corrected functionals.

GGA and METAGGA functionals might break the symmetry of the Bravais lattice slightly for cells that are not primitive cubic cells. The origin of this problem is subtle and relates to the fact that the gradient field breaks the lattice symmetry for noncubic lattices. This can be fixed by setting

GGA_COMPAT = .FALSE.

to apply a spherical cutoff to the gradient field. In other words, the gradient field, as well as the charge density are set to zero for all reciprocal lattice vectors $\bold{G}$ that exceed a certain cutoff length $\bold{G}_{cut}$ before calculating the exchange-correlation energy and potential. The cutoff $\bold{G}_{cut}$ is determined automatically so that the cutoff sphere is fully inscribed in the parallelepiped defined by the FFT grid in reciprocal space.

Mind: For compatibility reasons with older versions of VASP, the default is GGA_COMPAT=.TRUE. However, setting the tag usually changes the energy only in the sub-meV energy range (0.1 meV), and for most results the setting of GGA_COMPAT is insignificant. The most important exception is for the calculation of magnetic anisotropy, for which we strongly recommend GGA_COMPAT=.FALSE.

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