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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 $ 24\times 24 \times 24 $ grid points for small unit cells, and even for large unit cells one might need many k-points ( $ 6\times 6 \times 6$). 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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Next: Atoms Up: Examples Previous: Bulk calculations with internal   Contents   Index
N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at