GGA COMPAT: Difference between revisions

Revision as of 12:22, 18 October 2023

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.

Gradient corrected 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 ${\mathbf {G}}$ that exceed a certain cutoff length ${\mathbf {G}}_{cut}$ the gradient field as well as the charge density are set to zero 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 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.

@@ 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 flag restores the full lattice symmetry for gradient corrected functionals.
 ----
-For gradient corrected functionals the
+Gradient corrected functionals might break the symmetry of
-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.
+field breaks the lattice  symmetry for non-cubic lattices. This can be fixed by setting
-To fix this, a spherical cutoff is applied to the gradient field
+ GGA_COMPAT = .FALSE.
-for {{TAG|GGA_COMPAT}} = .''FALSE''.,
+to apply a spherical cutoff to the gradient field,
-e.g. for all reciprocal lattice vectors <math>\bold{G}</math> that exceed a certain cutoff length
+i.e. 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
+<math>\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>\bold{G}_{cut}</math> is determined automatically so that the cutoff sphere
-is fully inscribed in the parallelepiped defined by the FFT grid in
+is fully inscribed in the parallelepiped defined by the FFT grid in the reciprocal space.
-the reciprocal space.
-Since this flag restores the full lattice symmetry for gradient corrected functionals,
+{{NB|mind| For compatibility reasons with older versions of VASP, the default is {{TAG|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 {{TAG|GGA_COMPAT}} is set. The most important exception is for the calculation of magnetic anisotropy, for which we strongly recommend {{TAG|GGA_COMPAT}}{{=}}.''FALSE''.|:}}
-and we therefore recommend to set
- GGA_COMPAT = .FALSE.
-for all gradient corrected calculations. For compatibility reasons,
-the default is {{TAG|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
-{{TAG|GGA_COMPAT}} is set. The most important exception are magnetic
-anisotropies, for which we strongly recommend to set  {{TAG|GGA_COMPAT}} = .''FALSE''.
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