[gpaw-users] Calculating core-level shifts between different systems

[Eric Hermes]

Mathias,

I understand that to calculate the core binding energy for periodic 
systems it is sufficient and necessary to add an electron to the valence 
in the core-hole calculation. However, the issue is that in my 
understanding this binding energy is still relative to some 
system-dependent zero of energy at infinite separation. In non 
3D-periodic systems, this zero is well defined as the potential 
infinitely far away from the system, but for 3D-periodic calculations 
there is no such thing as infinitely far away from the system, so this 
reference becomes ill-defined. Because I am attempting to calculate core 
binding energy shifts, I need to correct for this value, and it is 
easier to think about for 2D-periodic systems, which are better suited 
for slab calculations anyway than fully 3D-periodic systems.

Do you believe that generating a core-hole setup for Pd would be 
problematic? I am unclear on the specifics of how setup generation works 
in GPAW. However, in VASP, the core-hole PAW pseudopotentials are 
generated on-the-fly during the course of the calculation from the 
ground-state pseudopotentials if ICORELEVEL=2 is specified. This allows 
one to perform a calculation in which the valence electrons relax in the 
presence of the core hole, but the other core electrons are fixed in 
their ground state configuration. Presumably this is also how GPAW would 
perform a core-hole calculation. Are there some differences that may 
cause problems that I should be aware of?

Thanks for your reply,
Eric

On 3/16/2015 10:21 AM, Mathias Ljungberg wrote:
> Hi Eric,
>
> On Mar 16, 2015, at 3:45 PM, Eric Hermes wrote:
>
>> Hello,
>>
>> I wish to calculate the core-level shift of the Pd 3d 5/2 state in PdO, PdO2, and Pd-Te intermetallics
>> with respect to metallic Pd in order to compare against experimental XPS spectra. The experimentalists indicate that the presence of Te in the intermetallic shifts the Pd 3d 5/2 state to higher binding energy, but my chemical intuition coupled with Bader analysis indicates the opposite. I have attempted to calculate these properties in VASP (using the ICORELEVEL tag), but the oxide shifts do not even semi-quantitatively agree with experiment (although they do agree qualitatively, as the shifts are ordered correctly). Part of the ambiguity here is that I am unsure what to take as the zero of energy in fully 3D periodic systems to properly compare binding energies.
>>
> I am not sure how well a 3d core hole would work in GPAW, you would have to try to create your own PAW dataset with such a core hole. Also there is no spin-orbit in GPAW.
>
>
>> I believe that GPAW might be better able to calculate these values, as I can do 2D-periodic slab calculations which have a true zero reference energy.
> I think this is not necessary. Just introduce an extra electron in the system to compensate for the core hole, this should work well for metals. You calculate the XPS position by total energy differences between the ground state and the state with a core hole (and an extra electron)
>
>> However, I am slightly confused by the information on this page:
>> https://wiki.fysik.dtu.dk/gpaw/tutorials/xas/xas.html
>> The citation Leetmaa2006 seems to be missing, and I cannot find what paper this refers to. I am unsure of what exactly is being calculated in the "Further considerations" section; is this how one compares to experimental XPS spectra? Can someone help guide me in doing these calculations within GPAW?
>>
> The tutorial mainly concerns XAS for small molecules. For molecules XPS should be computed by really removing the electron, but for a solid experience has shown that is it better add an electron to the system to preserve charge neutrality. For a metal the XPS is even experimentally referenced to the fermi level so there should be no problems there. I think the Leetmaa reference is not so useful for you anyway since it deals with liquid water. Maybe it should be removed from the tutorial...
>
> Best regards,
> Mathias Ljungberg
>
>> Thank you,
>> Eric Hermes
>>
>> -- 
>> Eric Hermes
>> J.R. Schmidt Group
>> Chemistry Department
>> University of Wisconsin - Madison
>>
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-- 
Eric Hermes
J.R. Schmidt Group
Chemistry Department
University of Wisconsin - Madison