View source for Supercell core-hole calculations

== Preparatory steps for SCH calculations ==
To run a successful calculation you have to make the following preliminary steps:

=== Make super cell for structure ===

To minimize the interaction between core holes from neighboring cells the super cell size has to be converged. The convergence is very material dependent and has to be in principle done every time for a new material. It's better to start bottom up from the small cell's. 
{{NB|mind|Although by increasing the cell size the k mesh is implicitely also increased it still has to be also converged since the spectrum can depend also very strongly on the k points.}}

=== Select one atom in the file that will carry the core-hole and provide a POTCAR file for that atom ===
After making the super cell one atom has to be made to a new species with a single atom in it that will carry the core-hole. The initial line for the number of atoms and atoms for example can look like this
 Mg O
 32 32
If we are for example interested in the K-edge spectrum of Mg, we would have to change the {{TAG|POSCAR}} file as follows
 Mg Mg O
 1 31 32
Since we create a new species this way we need the {{TAG|POTCAR}} information for it. This is very easily done by taking the {{TAG|POTCAR}} file for the same species an concatenating it to the {{TAG|POTCAR}} carrying all species: i.e. ''cat POTCAR_Mg POTCAR''.

The procedure for oxygen would be very similar:
 Mg O O
 32 31 1
and ''cat POTCAR POTCAR_O''.

{{NB|mind|One typical source of error is that the additional {{TAG|POTCAR}} is not added to the main {{TAG|POTCAR}} file or that the order of species is not the same in the {{TAG|POSCAR}} and {{TAG|POTCAR}} files.}}

{{NB|warning|It is strongly recommended to use the available GW PAW potentials for the {{TAG|POTCAR}} files, since many standard potentials don't have projectors with quantum numbers 2 or larger and the GW potentials are more exact for excited states than the standard potentials.}}

== Calculations ==
The supercell core-hole calculations (SCH) consist in principle of two steps:
*Self-consistent electronic cycle with core hole.
*Calculation of dielectric function of core electron with the band structure from the SCF run.

In the VASP implementation these two steps are all done in a single calculation.

'''Example INCAR file'''

An example input for the 2s K-edge of Mg in MgO would look like the following:
  {{TAGBL|CH_LSPEC}}=.TRUE
  {{TAGBL|CH_NEDOS}}=1000
  {{TAGBL|CH_SIGMA}}=0.3
  {{TAGBL|ICORELEVEL}}=2
  {{TAGBL|CLNT}}=1
  {{TAGBL|CLN}}=2
  {{TAGBL|CLL}}=0
  {{TAGBL|CLZ}}=1.0
  {{TAGBL|NBANDS}}=600

In the following the {{TAG|INCAR}} tags controlling the calculationare explained:

'''Main control tags'''
*{{TAG|ICORELEVEL}}: To enable core-hole calculations in the final-state approximation with self-consistent field cycles (SCF) one has to set {{TAG|ICORELEVEL}}=2. Core-hole calculations in the initial-state approximation ({{TAG|ICORELEVEL}}=1) are also available, but they are physically less relevant and should be only used if specially needed. 
*{{TAG|CH_LSPEC}}: To obtain X-ray absorption spectra (XAS) the following flag has to be set {{TAG|CH_LSPEC}}=''.TRUE.''.
*{{TAG|CLNT}}: This tag selects the species holding the core hole. This number corresponds to the species defined  in the {{TAG|POSCAR}} and {{TAG|POTCAR}} files. 
*{{TAG|CLN}}: Specifies the <math>n</math> quantum number of the excited electron.
*{{TAG|CLL}}: Specifies the <math>l</math> quantum number of the excited electron.
*{{TAG|CLZ}}: Specifies how much of a faction of the chosen electron should be excited. Usually one always sets {{TAG|CLZ}}=1.0, but in some cases values lesser than 1 can lead to better agreement with experiment. However, this should be handled with caution since the physics behind is very dubious.



'''Important''': One very important parameter that has to be increased in the calculation is {{TAG|NBANDS}}. It gives the upper range for which the spectrum can be calculated.

To check how many bands one has in advance VASP can be run in a dry mode which doesn't do any "actual" calculations but only does the setup up steps:
 vasp --dry-run 

The parameter {{TAG|CH_NEDOS}} sets the number of grid points on the energy axis of the spectrum.

The broadening of the spectrum is of Gaussian form and the broadening width in eV is set by {{TAG|CH_SIGMA}}. We recommend to use a very small broadening {{TAG|CH_SIGMA}}<math>\le</math>0.001 in the calculations and broaden the spectrum in post processing. Also the spectrum can be recalculated with different parameters without the need to redo the electronic self-consistent field cycle. For that one can use the converged {{TAG|WAVECAR}} from the previous calculation and set {{TAG|ALGO}}=''None'' together with the new paramters for the spectrum "CH_*" in the {{TAG|INCAR}} file.

'''Excited electron and supercell core hole calculations (XCH)'''

In the

== Output ==
The dielectric function is written to the following files:
*[[OUTCAR]]
*[[vaspout.h5]]
*[[vasprun.xml]]


Usually for an absorption spectrum all six components of the dielectric tensor are summed up. In most cases the obtained spectrum needs further processing via an energy dependent broadening.

=== OUTCAR ===

The freuqency dependent dielectric tensor, which is directly proportional to the absorption spectrum, is written to the {{TAG|OUTCAR}} file. It starts with the following lines:
   frequency dependent IMAGINARY DIELECTRIC FUNCTION (independent particle, no local field effects) density-density
     E(ev)      X         Y         Z        XY        YZ        ZX
  --------------------------------------------------------------------------------------------------------------
The energies of the excitations are with respect to the energy levels of the core electron of interest.
The start of the output of the dielectric function with respect to excitation energy is set slightly below the first peak to avoid many zeros over a large energy range, since core states have very large binding energies. 

=== vaspout.h5 ===
The energies of the excitations are with respect to the energy levels of the core electron of interest.

=== vasprun.xml ===
The energies of the excitations are with respect to the highest occupied bands (without the core hole).
----
[[Category:Linear response]][[Category:Dielectric properties]][[Category:XAS]][[Category:Howto]]