Band-decomposed charge densities

The partial (band-decomposed) charge density can be used to analyze the contributions of different orbitals or energy ranges to a specific region in real space. It helps in gaining insight and visualizing electronic, magnetic, or transport properties, and is especially important when simulating scanning tunneling microscopy (STM) pictures. In VASP, the calculation of partial charges is a quick post-processing step that is selected by setting LPARD = .TRUE. in the INCAR file. It is necessary to provide a WAVECAR file from a converged ground state calculation as an input file. To select the contributing k points and bands, various options exist, which can be selected via the NBMOD, IBAND, EINT, and KPUSE tags.

Mind: All charge densities, including the band-decomposed charge densities, are symmetrized using both the space and point group symmetries. However, in certain cases, such as when calculating partial charge from selected k-points, this can lead to wrong results. In that case, the symmetry must be turned off during the initial ground state calculation from which the WAVECAR is generated, as well as during the subsequent band-decomposed charge density calculation.

Input tags for selecting and writing the partial charges

The following list briefly explains the various INCAR tags that control the behavior of the band-decomposed charge density decomposition. Please refer to the documentation of each tag for further details.

• LPARD: Toggles the partial charge postprocessing on or off. If only this tag is set, the valence charge density is computed for all occupied bands and written to the CHGCAR file (without the augmentation occupancies usually written to that file).
• IBAND: An integer array specifying the bands to include in the partial charge density. If IBAND is specified, NBMOD is automatically set to the number of selected bands.
• EINT: Specifies an energy interval. Any energy bands with eigenvalues within this range will contribute to the calculation of the partial charge density. If the value of the NBMOD tag is set to -3, the energy values are interpreted as relative to the Fermi energy. If the NBMOD tag is not set or is set to -2, the provided energy values will be considered as absolute total energies.
• NBMOD: This tag controls the mode of selecting bands that should contribute to the calculation of partial charges.
  • NBMOD = n: Use n bands (set automatically if IBAND is set).
  • NBMOD = 0: Use all bands (occupied and empty).
  • NBMOD = -1: Use all occupied bands (and write to CHGCAR instead of PARCHG).
  • NBMOD = -2: To choose the bands that contribute, you can utilize an energy interval defined by the tag EINT.
  • NBMOD = -3: Use an energy interval relative to the Fermi energy to select contributing bands (defined by EINT).
• KPUSE: Specifies which k-points are used in the evaluation of the partial DOS.
• LSEPB: Specifies whether to write the partial charge density for selected bands to individual files or merge them.
• LSEPK: Specifies whether to write the partial charge density for selected k points to individual files or merge them.

Output files

The partial valence charge density is written in the PARCHG file. If you want to separate the output by k points or bands, setting LSEPB and/or LSEPK allows you to write it to multiple PARCHG.*.* files.

Mind: For spin-polarized calculations, the PARCHG and its variants hold the total density and the magnetization density. For instance, if the 4th band is selected (IBAND =4) the first data set in the PARCHG file corresponds to the summed density of the 4th spin up and 4th spin down orbital, whereas the second data set holds the difference between the 4th spin-up and 4th spin-down orbital (magnetization density). Hence, to obtain the charge density corresponding to a specific orbital of a specific spin channel some post-processing of the PARCHG file might be required (building differences or sums). A simple workaround is to use EINT and specify sufficient digits to select only one orbital from either the spin-up or spin-down channel.

Related tags and articles

LPARD, IBAND, EINT, NBMOD, KPUSE, LSEPB, LSEPK, PARCHG, Examples that use this tag

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