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

[Mathias Ljungberg]

On Mar 16, 2015, at 4:47 PM, Eric Hermes wrote:

> 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.
> 

Well, the truth is that the experimentalists don't measure XPS like that because the work function of a material is  dependent on the actual surface, and to avoid taking this into account one refers the XPS to the fermi level (at least for metals where there is a well-defined fermi level). So what you want to compute it the energy it takes to lift  the core electron to the fermi level which is exactly what the total energy difference Delta SCF or Delta Kohn Sham is accomplishing. See for example http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.081401  
 

> 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?
> 

In GPAW you need to construct your own PAW dataset or "setup" with a core hole. This means that the core electrons will adjust to the hole, but when it is put into the actual calculation only the valence electrons can relax. It is also possible to construct  setups with more electron in the valence but I don't think this is a good option in your case (there will be many electrons).

Best regards, 
Mathias

> 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
>