SCH calculations
The calculations 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. To run a successful XAS calculation the following steps have to be made:
1) 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.
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.
2) Select one atom in the POSCAR 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 POSCAR file as follows
Mg Mg O 1 31 32
Since we create a new species this way we need the POTCAR information for it. This is very easily done by taking the POTCAR file for the same species an concatenating it to the 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.
Mind: One typical source of error is that the additional POTCAR is not added to the main POTCAR file or that the order of species is not the same in the POSCAR and POTCAR files.