GGA COMPAT: Difference between revisions

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{{TAGDEF|GGA_COMPAT|.TRUE. {{!}} .FALSE. |.FALSE.}}
{{TAGDEF|GGA_COMPAT|.TRUE. {{!}} .FALSE. |.TRUE.}}


{\tt GGA\_COMPAT} = .TRUE. | .FALSE.
\begin{tabular} {lll}
Default \\
{\tt GGA\_COMPAT } & = &  .TRUE.
\end{tabular}\vspace{5mm}
\noindent
For gradient corrected functionals the  
For gradient corrected functionals the  
exchange correlation functional might break the symmetry of
exchange correlation functional might break the symmetry of
Line 16: Line 8:
field breaks the lattice  symmetry for non-cubic lattices.
field breaks the lattice  symmetry for non-cubic lattices.
To fix this, a spherical cutoff is applied to the gradient field
To fix this, a spherical cutoff is applied to the gradient field
for {\tt  GGA\_COMPAT = .FALSE.},
for {{TAG|GGA_COMPAT}} = ''.FALSE.'',
e.g. for all reciprocal lattice vectors $\bf G$ that exceed a certain cutoff length
e.g. for all reciprocal lattice vectors <math>\bold{G}<\math> that exceed a certain cutoff length
$G_{\rm cut}$ 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 is set to
zero before calculating the exchange correlation energy and potential.  
zero before calculating the exchange correlation energy and potential.  
The cutoff $G_{\rm cut}$ is determined automatically so that the cutoff sphere
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.

Revision as of 19:51, 15 January 2017

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

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 <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.

This flag restores the full lattice symmetry for gradient corrected functionals, and we therefore recommend to set \begin{verbatim}

GGA_COMPAT = .FALSE.

\end{verbatim} for all gradient corrected calculations. For compatibility reasons, the default is {\tt 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 {\tt GGA\_COMPAT} is set. The most important exception are magnetic anisotropies, for which we strongly recommend to set {\tt GGA\_COMPAT = .FALSE.}.