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# LMAXFOCK

**LMAXFOCK** = [integer]

Default: **LMAXFOCK** = 4

Description: **LMAXFOCK** sets the maximum angular momentum quantum number *L* for the augmentation of charge densities in Hartree-Fock type routines.

In the PAW method, the difference between the charge density of the all-electron partial waves and the pseudo partial waves

is usually treated on spherical grids centered at each atom
(one-center terms inside the PAW spheres, see PAW method). To describe long range electrostatic effects, the *moments* of the differences of the all-electron and pseudo charge density
also need to be added on the plane wave grid (compensation density, see PAW method).
These compensation charges exactly restore the moments of the all-electron density on the plane wave
grid. For the charge densities used in the Hartree and DFT term,
the augmentation is done exactly up to the maximum *L* quantum number required by the POTCAR files,
whereas for the Fock exchange, for reasons of efficiency,
the augmentation on the plane wave grid is controlled by **LMAXFOCK**.

Specifically, when the exchange energy is evaluated

the overlap density
between two Bloch orbitals needs to be calculated on the plane wave grid (see
PAW method). The tag **LMAXFOCK**
controls up to which *L* quantum number, the compensation charge is calculated on the plane wave grid (compared
PAW method compensation charge):

To accelerate convergence with respect to **LMAXFOCK**, VASP uses a small trick: the contributions from the Hartree-Fock one-center terms are evaluated for the pseudo orbitals also only up to *L*=**LMAXFOCK**, whereas the one-center terms for the exact all-electron orbitals are evaluated up to the maximum required *L* (twice the angular quantum number of the partial wave with the highest *l*). The default is **LMAXFOCK**=4, and it might be necessary to increase this parameter, if the system contains f-electrons. Since this increases the computational load considerably (about factor 2), it is recommended to perform tests, whether the results are already reasonably converged using the default **LMAXFOCK**=4.

To be compatible w.r.t. old releases, VASP also reads the flag HFLMAX to the same effect as **LMAXFOCK**.