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

[Eric Hermes]

Miguel,

Thank you for your quick reply. The way that I performed the calculation 
in VASP was to use 5 layers of the material (Pd, PdO, PdO2, and Pd3Te) 
fixed at their bulk geometry and a 15 angstrom vacuum gap. Because the 
slabs were symmetric, I did not use a dipole correction. I used 
ICORELEVEL=1 (the initial state approximation) with the primitive slab 
unit cell and a high K-point density. I produced the LOCPOT file with 
LVTOT=.TRUE., averaged the local potential over X and Y, and took the 
value of the local potential in the middle of the vacuum gap as my 
reference energy (Evac). The core level energies were computed as Evac - 
Ecore, where Ecore is the reported 3d state energy for the most buried 
Pd atom in my system. Relative to metallic Pd, I obtained a core level 
shift of 0.83 eV for PdO and 2.09 eV for PdO2, as compared to the 
experimental values of about 1.8 eV for PdO and 3.5 eV for PdO2. I also 
performed the same procedure in the Slater-Janak transition state 
approximation (ICORELEVEL=2, CLZ=0.5) with very large supercells of the 
slab and obtained shifts of 1.30 eV for PdO and 1.98 eV for PdO2.

Is there something about these procedures that is incorrect? For now I 
will try the same procedure with HSE06 and see if it produces values 
that are in better agreement with experiment.

Thanks,
Eric

On 3/16/2015 10:05 AM, Caro Miguel wrote:
> Dear Eric,
>
> You can also use a 3D slab calculation with VASP (or plane waves in GPAW) including dipole corrections along the direction perpendicular to your slab. The Hartree potential will converge to the vacuum level if you leave enough vacuum between periodic replicas of the slab, and this is your absolute reference (subtract that from every other number). In a non-periodic calculation you still need to leave enough vacuum to prevent interaction with the box edges.
>
> You can check out our recent paper, and especially the more standard references therein: http://scitation.aip.org/content/aip/journal/jap/117/3/10.1063/1.4905915
>
> Note that in order to get the correct positioning for your states you might need to resort to a scheme beyond GGA/LDA, that is try a hybrid functional or GW.
>
> Regards,
> Miguel
>
> Dr. Miguel Caro
> Postdoctoral researcher
> Department of Electrical Engineering and Automation, and
> COMP Centre of Excellence in Computational Nanoscience
> Aalto University, Finland
> Email: mcaroba at gmail.com
> Work: miguel.caro at aalto.fi
> Website: http://mcaroba.dyndns.org
>
> ________________________________________
> From: gpaw-users-bounces at listserv.fysik.dtu.dk [gpaw-users-bounces at listserv.fysik.dtu.dk] on behalf of Eric Hermes [ehermes at chem.wisc.edu]
> Sent: 16 March 2015 16:45
> To: gpaw-users
> Subject: [gpaw-users] Calculating core-level shifts between different systems
>
> 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 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. 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?
>
> 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