LMAXFOCKAE: Difference between revisions

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{{TAGDEF|LMAXFOCKAE|[integer]}}
#REDIRECT [[NMAXFOCKAE and LMAXFOCKAE]]
{{DEF|LMAXFOCKAE|-1|for Hartree-Fock and hybrid functionals |  4 | for post DFT methods}}
 
Description: {{TAG|NMAXFOCKAE}} and {{TAG|LMAXFOCKAE}} determine whether
the overlap densities in the Fock exchange and correlated wave function methods are accurately reconstructed on the plane wave grid. This flag generally only applies to the Fock-exchange part as well as many-body
post DFT methods (GW, RPA, MP2, etc.).
 
----
In the PAW method, the difference between the charge density of the all-electron partial waves and
the pseudo partial waves
<math>
Q_{\alpha\beta}(r)= \phi^*_\alpha(r)\phi_\beta(r)  - \tilde \phi^*_\alpha(r)\tilde \phi_\beta(r)
</math>
is usually restored on spherical grids centered at each atom
(one-center terms inside the PAW spheres). To describe long range electrostatic terms, the ''moments'' of the differences of the all-electron and pseudo charge density are usually
also added on the plane wave grid up to a certain ''l'' quantum number (see {{TAG|LMAXFOCK}}).
These augmentation charges restore the moments of the all-electron density on the plane wave
grid.
 
For the RPA, GW, and most post DFT methods, the one-center terms are presently,
however, not implemented. Depending on the material, this can cause sizable errors
in particular for 3d and (to a lesser extent) 2p, 4d and 5d elements.
To correct for this error, an alternative treatment is implemented
on the plane wave grid. This  allows to restore the all-electron densities accurately on the plane wave grid
instead of the one-center grids by specifying the flags {{TAG|LMAXFOCKAE}} and {{TAG|NMAXFOCKAE}}.
 
To achieve this improved treatment on the plane wave grid, <math> Q_{\alpha\beta}(r) </math> is Fourier transformed  to reciprocal space <math> Q_{\alpha\beta}(q) </math> and then expanded
in a set of orthogonal functions localized at each atomic site. These augmentation charges
are then added to the pseudo charge densities on the plane wave grid.
 
 
For {{TAG|LMAXFOCKAE}}=-1 (the default for DFT and Hartree-Fock calculations), only the moments of the all-electron charge densities are restored on the plane wave grid. This setting is exact for Hartree-Fock
since the one-center terms are implemented.
 
If {{TAG|LMAXFOCKAE}} is set to values larger than -1 (and {{TAG|NMAXFOCKAE}}=1), not only the moments of the all-electron charge density are restored, but also the all-electron charge density is restored up to a typical plane wave energy of 140 eV. This setting yields very accurate results for post DFT methods (MP2, RPA, GW, etc.) for most sp bonded materials.  {{TAG|LMAXFOCKAE}} is used to specify the maximum spherical (l) quantum number up
to which this more accurate treatment is used. The default is {{TAG|LMAXFOCKAE}}=4 for post DFT methods.
If no accurate augmentation is desired by the user, simply set {{TAG|LMAXFOCKAE}}=-1 in the INCAR file.
 
If {{TAG|LMAXFOCKAE}} is set to values larger than -1 and {{TAG|NMAXFOCKAE}}=2, the charge density is restored accurately on the plane wave grid up to a typical plane wave energy of 380 eV. As before,  {{TAG|LMAXFOCKAE}} can be used to specify the maximum spherical (l) quantum number up
to which this more accurate treatment is used.  {{TAG|NMAXFOCKAE}}=2 yields very accurate results for
post DFT methods (MP2, RPA, GW) even for difficult 3d elements. For RPA and MP2 total energy calculations, differences between {{TAG|NMAXFOCKAE}}=1 and {{TAG|NMAXFOCKAE}}=2 are usually tiny for total energy differences. Since the absolute correlation energies might change, it is vital to use the same setting for
{{TAG|NMAXFOCKAE}} and {{TAG|LMAXFOCKAE}}, if energy differences are calculated.
For GW calculations, increasing  {{TAG|NMAXFOCKAE}} from 1 to 2 might change QP energies by 100-200 meV for 3d and late 4d and 5d elements.
 
 
The setting for {{TAG|LMAXFOCKAE}} should be also considered carefully. Generally, it suffices to set {{TAG|LMAXFOCKAE}} to twice the maximum ''l'' quantum number found in the {{FILE|POTCAR}} file.
For instance for sp elements, {{TAG|LMAXFOCKAE}} = 2 suffices. For d elements, {{TAG|LMAXFOCKAE}} = 4 suffices
(a d electron can create charge densities with ''l''-quantum numbers up to 4), whereas for f elements, users
should test whether  {{TAG|LMAXFOCKAE}} = 6 is required.
 
In summary, usefully manual settings of {{TAG|NMAXFOCKAE}}  and {{TAG|LMAXFOCKAE}} are:
*  {{TAG|LMAXFOCKAE}}=-1, to switch off the accurate augmentation altogether
* {{TAG|LMAXFOCKAE}}=4 (or larger) to force an accurate treatment for the HF part even in Hartree-Fock calculations
* {{TAG|NMAXFOCKAE}}=2, to select the very accurate augmentation. Please check whether the VASP default setting for {{TAG|LMAXFOCKAE}} suffices (OUTCAR file).
 
 
== Related Tags and Sections ==
{{TAG|LMAXFOCK}}, {{TAG|NMAXFOCKAE}}, {{TAG|QMAXFOCKAE}}
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[[The_VASP_Manual|Contents]]
 
[[Category:INCAR]][[Category:Hybrids]][[Category:GW]]

Latest revision as of 10:20, 28 February 2025