GGA COMPAT: Difference between revisions

Latest revision as of 05:50, 20 October 2023

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 ${\mathbf {G}}$ that exceed a certain cutoff length ${\mathbf {G}}_{cut}$ before calculating the exchange-correlation energy and potential. The cutoff ${\mathbf {G}}_{cut}$ 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.

@@ Line 2: / Line 2: @@
 {{TAGDEF|GGA_COMPAT|.TRUE. {{!}} .FALSE. |.TRUE.}}
-Description: This flag restores the full lattice symmetry for gradient corrected functionals.
+Description: If set to {{TAG|GGA_COMPAT}} = .''FALSE''., this tag restores the full lattice symmetry for gradient-corrected functionals.
 ----
-For gradient corrected functionals the
+{{TAG|GGA}} and {{TAG|METAGGA}} functionals might break the symmetry of
-exchange correlation functional might break the symmetry of
+the Bravais lattice slightly for cells that are not primitive cubic cells.
-the Bravais lattice slightly for non cubic cells (this
+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
-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 {{TAG|GGA_COMPAT}} = .''FALSE''.,
-e.g. for all reciprocal lattice vectors <math>\bold{G}</math> that exceed a certain cutoff length
-<math>\bold{G}_{cut}</math> the gradient field as well as the charge density is set to
-zero before calculating the exchange correlation energy and potential.
-The cutoff <math>\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.
-Since 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,
+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 <math>\bold{G}</math> that exceed a certain cutoff length
-the default is {{TAG|GGA_COMPAT}} = ''.TRUE.'' until VASP.5.2.
+<math>\bold{G}_{cut}</math> before calculating the exchange-correlation energy and potential.
-However, setting the flag usually changes the energy only in the sub meV energy range
+The cutoff <math>\bold{G}_{cut}</math> is determined automatically so that the cutoff sphere is fully inscribed in the parallelepiped defined by the FFT grid in reciprocal space.
-(0.1 meV), and for most results it does matter little how
+{{NB|mind| For compatibility reasons with older versions of VASP, the default is {{TAG|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 {{TAG|GGA_COMPAT}} is insignificant. The most important exception is for the calculation of magnetic anisotropy, for which we strongly recommend {{TAG|GGA_COMPAT}}{{=}}.''FALSE''.|:}}
-{{TAG|GGA_COMPAT}} is set. The most important exception are magnetic
-anisotropies, for which we strongly recommend to set  {{TAG|GGA_COMPAT}} = .''FALSE''.
 == Related tags and articles ==
 {{TAG|GGA}},
@@ Line 36: / Line 19: @@
 {{sc|GGA_COMPAT|Examples|Examples that use this tag}}
 ----
-[[The_VASP_Manual|Contents]]
-[[Category:INCAR tag]][[Category:Exchange-correlation functionals]][[Category:GGA]]
+[[Category:INCAR tag]][[Category:Exchange-correlation functionals]][[Category:GGA]][[Category:Symmetry]]