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##

Accurate DOS and Band-structure calculations

Calculating a DOS can be done in two ways: The simple one is to
perform a static (`NSW=0`, `IBRION=-1`) selfconsistent calculation and to use
the DOSCAR and vasprun.xml file from this calculation. The vasprun.xml file
can be visualized using p4v.

The simple approach discussed above is not applicable in all
cases. A high quality DOS might require very fine k-meshes up to
grid points for small unit cells, and even for
large unit cells one might need many k-points (
).
Similar problems occur for band-structure calculations, where one needs to calculate
the eigenvalues along certain high symmetry lines in the BZ
(at least 10 k-points are required for reasonably results).

Since, the charge density and the effective potential converge
rapidly with increasing number of k-points, it is often helpful to calculate
the selfconsistent charge density using a few k-points.
In the second step, a non-selfconsistent calculation using the precalculated CHGCAR
file from the selfconsistent run (i.e. `ICHARG=11`, see section 6.15)
can be performed (applicable only to density functional theory calculations,
however).

For `ICHARG=11` and density functional theory calculations, all k-points
become essentially independent, because the charge density and the potential are
kept fixed.
If necessary it is even possible, to split up the k-points and
calculating the eigenvalues individually for each k-point, although with present
computing platforms this is rarely required.

For hybrid functionals and Hartree-Fock, the band structure can be calculated
by adding additional k-points with zero weight to the KPOINTS file. This is easily
achieved, by performing first a standard hybrid functional calculation with
a conventional KPOINTS file. After the run, copy the IBZKPT file
to the KPOINTS file (this file stores explictly the list of k-points used
in the previous calculation), and simply add the desired additional k-points
with zero weight. Since VASP uses an iterative matrix diagonalization
and since the added k-points do not influence the energy, one needs
to force VASP to perform at least 5 iterations before inspecting
the one-electron energies at k-points with zero weight
(`NELMIN = 5`).

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N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at