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Hello,
I am interested in determining the Fermi level of a material with respect to vacuum (or the work function in the case of insulators).

I see the VASP output posts a value for the Fermi level, however it is positive. This leads me to believe this value is not with respect to the vacuum. Is this true? If so, what is this value with respect to? And how can I get the Fermi level with respect to the vacuum?

Thank you!

Hi,

for 3D there is no outside reference, because there is just no vacuum around. For 2D (slab) calculations you have to look at the potential in the vacuum region (you need the LOCPOT file for that). Substract or add (I forgot the sign convention used here) and there it is.

Alex

1. Do you mean to run the same type of calculation (e.g. LDA,PBE, Hybrid DFT) I am interested for the bulk but on a slab?
2. And, Can you please clarify what to add/subtract?
Thank you

1. yes. There can _not_ be a vacuum in a bulk calculation.
2. In the file LOCPOT the potential V(x,y,z) on every grid point is plotted. So you go to the grid points of your vacuum region of your slab calculation, make an average V(z) (assuming z is the direction perpendicular to the surface). Then you take this V(z) and add (or substract it) from your Fermi energy.

Of course, this is nor exactly right, because here is a potential and there an energy. So you'd have to multiply by a test charge 1e to get the energy of the test charge in the vacuum. But the numbers stay the same ...

Cheers,

Alex