[Wien] core-hole calculation in a molecule

[Pavel Ondračka]

On Wed, 2019-06-19 at 16:25 +0200, Peter Blaha wrote:
> 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.

I will try the Δ-SCF approach as well to see if it behaves differently.
But still, I've now done a lot of similar calculations and there was
always some dependency on the cell size so this is a really big
surprise...

BTW for Δ-SCF "E-tot(N) - E_tot(N-1)" is not enough, also μ is needed,
which surprisingly no manuals mention...

> 
> 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.

Right, the problem is that ultimately I would like to do the
interaction with a surface as well (and look for changes), so I still
do need a periodic boundary condition. In general I agree, when
hydrogen comes into play the lapw approach is super slow... For now I'm
just exploring this so burning some extra CPU time is not an issue if
it ultimately saves me the troubles of learning yet another DFT code.

> 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.

I'm actually hoping to use core electron binding energy of atoms far
from the surface (in both the bulk and the molecule) as a reference to
check the core electron binding energy shifts of atoms directly at the
surface, but dunno how this will work in reality.

Thanks for all the feedback on the list (and in off-the-list emails
I've received as well)
Pavel