[Wien] core-hole calculation in a molecule

[Pavel Ondračka]

Dear Wien2k mailing list,

I'm trying to calculate core electron binding energies using the
Slaters transition state approach (half electron removed from the core
compensated by the background charge) in an organic molecule.

As part of the usual convergence checking I did four calculations with
different amount of vacuum, with 5Å, 10Å, 15Å and 20Å in all directions
in order to get some trend and try to extrapolate the final values.
This is the approach similar to what I use for insulators (increasing
supercell size), to estimate the supercell size error due to the
Coulomb interaction between the periodic images of the charged atom.
However to my first surprise there is no change in the binding energies
(~0.01 eV) observed. Thinking about it more it makes sense though, as
there is no screening in the vacuum, so there probably is no reduction
of the interaction (like in the simple electrostatic example where the
electric field intensity next to the infinite charged plane doesn't
depend on the distance to it).

I'm looking for an advice whether someone already tried something like
this and if this kind of calculation (i.e., corehole for molecule,
single atom, or even a 2D material) actually makes a sense from the
physical point of view and also within the lapw framework... For now
I'm comparing the relative shifts of the core electron binding energies
of different carbon atoms within the molecule, and the results looks
quite in agreement with the literature. However I'm not sure how much I
can trust the results and if I can actually compare the values also
with bulk materials.

Any advice would be appreciated
Best regards
Pavel