[Wien] core-hole calculation in a molecule

[Pavel Ondračka]

So just some brief follow up, in case someone finds this interesting.

First of all I've made a mistake in my previous calculations, there
actually is some dependence on the supercell size for the Slater's
transition state approach. However the difference in binding energies
is only ~0.05-0.1eV when going from 5Å vacuum to 10Å vacuum and changes
less than ~0.01eV when going further to 15Å...

I've tried the Δ-SCF approach as well and this is much worse. The
difference in binding energies is ~0.3-0.4eV when going from 5Å vacuum
to 10Å vacuum and changes by another ~0.2eV when going further to
15Å...

The absolute energy values are better for the Δ-SCF approach (by approx
0.5eV), but since we are about 5eV from the absolute experimental
values anyway, this is likely meaningless. For example taking LDA
instead of PBE can change the absolute values by > 2eV.

What is important, the relative shifts between different carbon atoms
(with respect to experimental data) are also better for the Slater's
transition state than for the Δ-SCF approach (with Slater's transition
state I can get around 10% difference from experiment, while for Δ-SCF
it is more like 20%).

In general I'm very happy with the calculations now, except for the
speed ;-)

Best regards
Pavel


On Fri, 2019-06-21 at 07:39 +0200, Pavel Ondračka wrote:
> 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
> 
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