[Wien] core-hole calculation in a molecule

[Peter Blaha]

This is certainly interesting.

For a molecule an alternative is to remove one electron and then use 
E-tot(N) - E_tot(N-1) as binding energy. However, in this case due to 
the charged cells, I'd expect quite some dependency on the cell size and 
some correction might be necessary.

Your findings indicate that Slater's transition state method is much better.

On the other hand: If you really want to do only organic molecules (but 
many of them), any non-periodic molecular code (eg. NWChem, which is 
free) will be MUCH cheaper and faster.

Your last question, comparison to bulk materials, you have to find out 
yourself.
I would not expect perfect agreement with experiment in all cases, 
simply because of the problem having a common Energy-zero (we use EF for 
this, but EF is well defined only in metals, but the VBM of an insulator 
or the HOMO of a molecule is not the same "Fermi energy".

Suppose you put a molecule far away from a metal surface, the DFT 
simulation will give you a common EF (which is most likely not where the 
HOMO of the molecule is). Thus   (E-1s - EF) will be different if you do 
a combined system or the molecule alone, even when the molecule is so 
far away that it behaves as a free molecule.

On 6/19/19 2:57 PM, Pavel Ondračka wrote:
> 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
> 
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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