Discrepancy between Fermi levels and electrostatic potential from LOCPOT for heterostructure?

[sophie_weber1]

I thought I would start over with a simpler question since I think I scared everyone away with my last Related to the same topic, I am confused about how to reconcile two difference outputs I have from the same calculation for an insulator-heavy metal heterostructure with vacuum. If I calculate the layer-projected density of states for the heterostructure, I see that in the center of the insulator (Cr2O3) region of the slab, the highest occupied states are below the highest occupied states in the center of the heavy metal (Pt), which sit at the heterostructure Fermi level, marked by the dashed black line. Then I would expect electrons to flow from Pt to Cr2O3 at the interface to try to equilibrate chemical potentials (and indeed, this is what looks to happen for the layers near the interface).

If I plot the electrostatic potential as a function of distance along the slab, with LVTOT=.TRUE. however, I see that the average electrostatic potential (marked by the red line) in the Cr2O3 part of the slab is higher/less negative than the average electrostatic potential in the Pt region of the slab. Since VASP plots the electrostatic potential with respect to electrons, this seems to imply that electrons should flow from Cr2O3 (higher potential) to Pt (lower potential) at the interface. So the exact opposite of what is implied by the density of states. It seems to me like if the highest occupied states in the Cr2O3 region of the slab are lower in energy than in the Pt region as indicated by the DOS, this has to mean that the electrostatic potential of the Cr2O3 region is lower in energy (more tightly bound).

If you can explain how I should understand this apparent discrepancy in the output, I would greatly appreciate it. Also, I'm curious as to what the reference level is set as for the LOCPOT, since it seems that some areas of the Pt potential are actually above zero; perhaps this is because I've included the exchange-correlation energy?

I attached the input files and the image showing the layer-projected density of states and the LOCPOT output which to me seem to contradict each other.

[pedro_melo]

Dear Sophie,

Calculations with heterostructures are not my speciality, so I will try to help as much as I can.

First I need to ask if you followed some of the recommendations written here. It does say that you should avoid plotting the exchange-correlation potential in LOCPOT, and that you should instead set LVHAR=T. From what you wrote you have included the exchange-correlation part. Could you check how the results change without it?

Then, I think that it might be helpful to also look at both the PDOS and the planar averages of the potential of the the isolated structures of Pt and Cr2O3. I do not know where the surface states of Cr2O3 are localised in energy, as you only show results for the interface. This might make it easier to understand how the transfer of electronic density compares to the expected behaviour.

Finally, I think that the values in LOCPOT are not computed with respect to any reference value. This is an issue with all DFT calculations, and that is why when comparing results for isolated Pt and Cr2O3 you should align the vacuum levels, as these are the best reference points.

Let me know if this helps. Kind regards,
Pedro

[sophie_weber1]

Hi Pedro,

Thanks a lot for your detailed response. I am currently running calculation with LVHAR=T rather than LVTOT, so I will see how this changes things.

As for your suggestion of looking at the pdos for the isolated structures, do you mean for the bulk structures, or for the same slab geometries of isolated Cr2O3 and isolated Pt with vacuum? Could you clarify a bit how this analysis could shed light (in particular, how should I compare the planar averages in the two cases)

And finally, if after all this it still seems that the electron transfer at the interface is from Pt to Cr2O3, and the electrostatic potential for electrons still seems to be higher in Cr2O3 than in Pt, do you agree that this seems contradictory? Or am I not thinking about it correctly the two facts can coexist?

Thank you again for your help,
Sophie